The cool thing about this idea is that it would really shape the experience of partying with your friends. You would actually get used to their custom songs as the normal song for that spell. It would be strange switching to a pickup party or getting a new bard and hearing completely different music during the battle. lol I love this idea.
On top of this, it would be cool to have a unique custom melody for each and every song played, so that people that party with you regularly can know what you are casting just from the audio.
I'm also completely fine with giving everyone the ability to play instruments, but reserving the accompanying magical effects to the Bard class. This is indeed something that would distract the masses for a while (especially the all lvl 50s people complaining about standing around in town with nothing to do)
