This is an optional parameter and is not supported by all clients. This parameter specifies the URL of the sound/music file. This allows downloading files from the MUD server. Client should always look in local directories first, and only download the file if it's not available locally. Each client supporting this option should support at least the HTTP protocol. Other protocols can be supported as well. The URL of the sound file is determined by combining the value of this parameter and fName. E.g., if fName is combat/win1.wav and URL is http://www.example.net:5000/sounds/, the URL of the remote file is http://www.example.net:5000/sounds/combat/win1.wav. The '/' character should be appended to U param if not given. The URL can also be enclosed in quotes (").
It is possible to specify default URL for all requests, that will be used in all subsequent request without the need to specify it in every trigger. This can be accomplished with a special type of trigger, with fName=Off and U param given. This URL will then be used as a default URL. See below for an example.
Clients should provide an option to allow users to turn off sound downloading, since this can have a significant impact on MUD playing performance. The original intent of MSP was to require the MUD to bundle its sound files and to require the player to download and unpack the sounds used by the MUD before playing. The U parameter was added later to allow on-the-fly downloading of sounds for players with fast network connections.
Possible values: URL string
Default value: No URL, uses previously set default URL. If no default is set, sound is not downloaded
Used in: SOUND, MUSIC
From the specification: