As far as I know, this will never be possible, you should hack the whole game, or have the source of the game where you want to do this for. Because the xbox cant run more then 1 programs....
I don't think you'll ever see this idea come to fruition. However, what you can do is create a simple audio mixer circuit and play a cd/mp3/whatever with your xbox audio. Just turn the in-game music off and let your music play.