It's in the TDA rules and other official rules that music, and for that matter other devices that can make noise must be inaudible to other players.
As to why, I would imagine it's there as a respect to other players so as to not distract them from decision making. You'll probably find it's just a blanket ban so that 1 in every 10 player who might complain about it won't have to because it's just not allowed. Plus music is a deeply personal thing, as in some people passionately dislike or love a certain type of music, et cetera. Its likely it is a ban to avoid confrontations/disagrees and the likes.
That would be my educated guess, but as to a 100% reason, I'm just basing this on other rules in poker, mainly about making the game a level, fair playing ground. If you remove an element completely no one can complain about it from a gameplay point of view.