What I imagine is instead of custom music for your own client like there already is, there be an item that lets you play a song for the entire duration of your match that the entire server can hear.
Right now you're probably coming up with all the arguments against this, and I know what they are too. Instead of two players having songs going at once, maybe only the player in the Tori position will have their song played. I believe this is how it works for displaying floor textures, so it is possible. Setting a volume limit for each song seems like a good idea to prevent someone from being overly annoying with their song of choice. And for those who are against turning music and sound volume off, adding a command and/or an option that mutes the song of someone or all people might be nice.
The only problem I foresee having no solution is the filesize. Audio files aren't necessarily the smallest out there.

Any thoughts or additions?

That's what the music item was supposed to do. Whoever was in the tori position, their music would play.

Thing is it's broke, and has been pretty much since day 1.

i think you should fix it

just saying