One of the features I try to include in my games is an in-game mp3 player. So while walking around, the player can open up their inventory, turn on their in-game music player (an actual item), and play whatever songs they want from their PC's directory.
It's fun.
This... blew my mind. I am absolutely most definitely going to do this.
As for the OP's question, I was considering the same thing, but had thought up a legal safety net to prevent me from being attacked by insane copyright mafia thugs. I would just label the 'mood' of each track, like ''high tempo dramatic music" or some such nonsense, then in whatever forum the game is delivered in (website, app store, whatever) the first comment would be a list of suggested tracks posted by a helpful anonymous player.
It's actually very sad that I came up with some sort of scheme for doing something that should be perfectly legal, and is definitely totally ethical. IANAL and the fear of litigation/terrorists/animal-flu/boogeymen just keeps me in my cage.
Any IP lawyers able to answer? Umm... IP lawyers browsing the developer forums at TIGSource...?