this library doesn't provide neither easy nor copyright-safe usage

as per #1 , sampling+ is a bad license choice , but perhaps even more importantly there is no easy way to use it . there is no single complete list of contributors that needed to be attributed per license requirement in every song that uses these samples ; failing to do so exposes one to law-suites (however good-willed original authors of samples were , their heirs might be a different story and with current fucked up copyright laws you'll only get in the clear 70+ years later) at any moment . furthermore , any changes made after initial release by original author violate sampling+ license , and by proxy would any composition using them (even if some form of attribution given is deemed acceptable) . i'm not a lawyer , so i'm not sure if these violations can be prosecuted without authors' consent , but again , their "intellectual property" heirs may decide it's worth extorting billions dollars from free culture enthusiasts

This is all true. Furthermore, according to the license, "appropriate credit" is defined as:

If supplied, you must provide the name of the creator and attribution parties, a copyright notice, 
a license notice, a disclaimer notice, and a link to the material. CC licenses prior to Version 4.0 
also require you to provide the title of the material if supplied, and may have other slight differences.

That means you'd need a license file to be distributed with your music, but that can easily get removed or lost, which means you need to put it in the meta tags. That means a copyright notice bigger than the one in this actual project's license file.

Containing Sonatina Symphonic Orchestra <https://github.com/peastman/sso> which is distributed
under the terms of the Creative Commons Sampling Plus 1.0 license
<https://creativecommons.org/licenses/sampling+/1.0/>

Problem is, there is no copyright notice in the license. The copyright holder didn't add a "Copyright John Smith 2011", so do we just assume it's Mattias Westlund 2011? What about the other contributors to the project since then? Missing those and using any of their contributions or modifications would be a copyright infringement without attributing them.

There is actually no legally safe way to use it whatsoever, and no matter how you distribute your music, you can be sued by anyone who acts on Mattias Westlund or the other contributors' behalf. I don't think you can be sued by random third parties, since the license says nothing about that I believe (as in, unlike the GPL iirc) but your idea of heirs etc is correct. You can basically be extorted.

There is also of course the prohibition on advertising clause which means that if you have a bandcamp or a youtube or etc, and you have music on it that uses SSO and music that doesn't, you're advertising one with the other, and are in breach of the license.

There is this:

e. Attribution and Notice
[...] You must keep intact all copyright notices for the Work and give the Original Author credit 
reasonable to the medium or means You are utilizing by [...]

It could be argued in court that "reasonable" does not cover a license whose conditions cannot even be met, but who wants that hanging over them?

I think a warning should be added to the readme.

I don't think that's actually the effect of the license. It doesn't cover the samples (which came from other sources as described in the README), only the sfz scripts used to control them. Attribution is only needed for derived works that remix the scripts, which is to say, other sample libraries derived from this one that reuse the scripts. Music created with this library doesn't contain any part of the scripts, so the license doesn't restrict it.

I agree it's still not the best choice of license, but I don't think it's that much of a problem.

Unfortunately the license info at the bottom of the readme is not clear enough to constitute actual free use, so you'd have to track down the actual original person who recorded or paid for the recordings to happen and find where they gave or didn't give permission. In the meantime I'm just using the samples directly from UIEMS. (I only just realised the samples actually came from here, I switched to UIEMS when I found out about the license issue without knowing that's where it came from originally haha...) They are not cropped or modified, just raw .aiff files, so they need editing to be useful in music production, but at least they gave explicit permission: "...may be downloaded and used for any projects, without restrictions"

I found out later that indeed I was confused when I read the legal code, since they mix "works", "derivative works" and "collective works" together.

Edit: To clarify though, the edited UIEMS samples are now under the CC license, NOT the license given by UIEMS, so if you use the SSO version of the UIEMS samples, you're still having to use them under the CC license, so be aware of that. Technically, you would have to edit the originals yourself to not be under that license.

The following text is a footnote on the deed:

If supplied, you must provide the name of the creator and attribution parties, a copyright notice, a license notice, a disclaimer notice, and a link to the material. CC licenses prior to Version 4.0 also require you to provide the title of the material if supplied, and may have other slight differences.

However, the licence itself sets slightly different and more specific requirements for attribution:

If You distribute, publicly display, publicly perform, or publicly digitally perform the Work or any Derivative Works or Collective Works, You must keep intact all copyright notices for the Work and give the Original Author credit reasonable to the medium or means You are utilizing by conveying the name (or pseudonym if applicable) of the Original Author if supplied; the title of the Work if supplied; to the extent reasonably practicable, provide the Uniform Resource Identifier, if any, that Licensor specifies to be associated with the Work or a Derivative Work, unless such Uniform Resource Identifier does not refer to the copyright notice or licensing information for the Work; and in the case of a Derivative Work, provide a credit identifying the use of the Work in the Derivative Work (e.g., "Remix of the Work by Original Author," or "Inclusion of a portion of the Work by Original Author in collage"). Such credit may be implemented in any reasonable manner; provided, however, that in the case of a Derivative Work or Collective Work, at a minimum such credit will appear where any other comparable authorship credit appears and in a manner at least as prominent as such other comparable authorship credit.

and

You may distribute, publicly display, publicly perform or publicly digitally perform the Work only under the terms of this License, and You must include a copy of, or the Uniform Resource Identifier for, this License with every copy or phonorecord of the Work or Derivative Work You distribute, publicly display, publicly perform, or publicly digitally perform. You may not offer or impose any terms on the Work that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder. You may not sublicense the Work. You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute, publicly display, publicly perform, or publicly digitally perform the Work with any technological measures that control access of use of the Work in a manner inconsistent with the terms of this License. The above applies to the Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Work itself to be made subject to the terms of this License. Upon notice from any Licensor You must, to the extent practicable, remove from the Derivative Work or Collective Work any reference to such Licensor or the Original Author, as requested.

If I read this correctly (for if you use SSO in music):

"You must keep intact all copyright notices for the Work" - in this case unclear as there is no proper copyright notice.
"give the Original Author credit reasonable to the medium or means You are utilizing by conveying the name (or pseudonym if applicable) of the Original Author if supplied;" - credit Mattias Westlund
"the title of the Work if supplied;" - say you use SSO
"to the extent reasonably practicable, provide the Uniform Resource Identifier, if any, that Licensor specifies to be associated with the Work or a Derivative Work, unless such Uniform Resource Identifier does not refer to the copyright notice or licensing information for the Work;" - link to SSO, possibly just https://github.com/peastman/sso, however as there is no copyright notice, it does not refer to it, but it does refer to licensing information.
"and in the case of a Derivative Work, provide a credit identifying the use of the Work in the Derivative Work (e.g., "Remix of the Work by Original Author," or "Inclusion of a portion of the Work by Original Author in collage")." - I believe this means you have to say that you use samples from SSO by Mattias Westlund in your music.
"Such credit may be implemented in any reasonable manner;" - don't like hide the credit somewhere.
"provided, however, that in the case of a Derivative Work or Collective Work, at a minimum such credit will appear where any other comparable authorship credit appears and in a manner at least as prominent as such other comparable authorship credit." - I believe this means that if you use SSO samples, and you credit other samples, you should put them in the same place, in the same way. It could, however also mean something more scary, that you should put the credit in the same place and way as the credit for you yourself creating the music with the samples (imagine having to add Mattias as a main artist on Spotify, like A Song - John Smith and Mattias Westlund - I hope not)
"You may distribute, publicly display, publicly perform or publicly digitally perform the Work only under the terms of this License" - doesn't apply, I believe, as it says "the Work" and not "Derivative Work", so don't redistribute SSO itself under a different license. -" You must include a copy of, or the Uniform Resource Identifier for, this License with every copy or phonorecord of the Work or Derivative Work You distribute, publicly display, publicly perform, or publicly digitally perform." - include the entire license or just a link to https://creativecommons.org/licenses/sampling+/1.0/ or something
"You may not offer or impose any terms on the Work that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder." - again, shouldn't apply, but don't like ask for money for a copy of SSO or something like that?
"You may not sublicense the Work." - Don't licence out SSO to someone else, they should licence it from Mattias (I believe this also only applies SSO itself, not any derivative work, because of the "the Work")
"You must keep intact all notices that refer to this License and to the disclaimer of warranties." - I don't believe this applies to derivative works, but if you distribute SSO, don't remove any notices that refer to the licence.
"You may not distribute, publicly display, publicly perform, or publicly digitally perform the Work with any technological measures that control access of use of the Work in a manner inconsistent with the terms of this License." - again, shouldn't apply, but don't like ask for money for a copy of SSO or something like that (again?)
"The above applies to the Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Work itself to be made subject to the terms of this License." not applicable, but if you include a full copy of SSO in something, you should (as above) not ask for money for the copy of SSO, but you can ask for money for other parts, you need to include the licence notices and such with SSO, but not other parts that aren's SSO, you can sublicence other parts, but not the SSO ones...
This is also why I don't believe the "You must keep intact all notices that refer to this License and to the disclaimer of warranties." applies to derivative works.
"Upon notice from any Licensor You must, to the extent practicable, remove from the Derivative Work or Collective Work any reference to such Licensor or the Original Author, as requested." - If Mattias doesn't want his name on your music, you should remove it if you can.

Apart from the copyright notices being unclear, I believe including a notice like "Including samples from Sonatina Symphonic Orchestra (https://github.com/peastman/sso) by Mattias Westlund. (https://creativecommons.org/licenses/sampling+/1.0/)" is acceptable. (I mean, like if you're distributing your music to streaming services this might be hard or even impossible, but if you can include a small amount of arbitrary text where you're putting up your music it should be fine.)

The advertising bit is scary though. Definitely don't use SSO in music for an advertisement or trailer or something, but sadly advertising is not properly defined so the problem described by SumianVoice is still there.

If anyone sees anything I interpreted incorrectly, could you please notify me?

I think you're reading much too much into it. :) I personally don't care what you do with the music you create, and while I can't speak for Mattias, I really doubt he cares either. If you're creating your own instrument library and you want to include some of these instruments in it, you need to credit where they came from. That's the intent of the license. But if you're just creating music with them, as far as I'm concerned that means you're using them in the way they're intended to be used.

If you're looking for specific guidelines, here is what I would do in your place. If you create music with SSO, anywhere you display your own copyright notice for the music also add a line, "Created with Sonatina Symphonic Orchestra" and the URL of this repository.

peastman / sso

this library doesn't provide neither easy nor copyright-safe usage #4