It'd be really neat if a mod's music discs could display what number a song is on an album.

Musics.jsons are limited to Minecraft's music:
https://github.com/KosmoMoustache/MusicNotification/blob/1.20.1/src/main/java/net/kosmo/music/ClientMusic.java#L84-L89

Version 1.6.0 allow mods and ressource pack to use musics.json. See the new file schema here.
The key identifier can be ignore