The super-first attempt could be a midi file with only one track, with 3 notes, like C, D, E.
In the beginning we can see in plain text on the web page:
the midi events and the note of these events (MIDI have 2 events per note, one when the note starts, and one when the note ends) [we will only use the event "note start", since the music box doesn't have a note duration ^_^ ]
the timestamp of these events (when they happens in the "timeline")
then...
with a graphic library (i think SVG could be a good first attempt, 'cause is well supported by browsers) put some circle inside a rectangle, on the screen!
A first attempt at creating some software requirements for discussion
Required
Suggested
Nice to have
User can open/import a MIDI file
User can export to a file format suitable for laser cutting