It is complicated, you will need to modify the client to do it, the music is packed so you will need a unzip utility specific for the game packs and find the exact song you want to swap, and then repack it. You can find some tutorials on how to do it @ youtube, but I can't say if it is considered a grave client modification.
Doable? yes, but it will take a while and it may get erased/replaced every time the Client updates