I'm a professional audio engineer, and I've done post-production work for several video games (along with movies and tv shows and other things as well), and I gotta tell you guys... this isn't as easy as you want it to be.
I highly recommend just learning to love the music that's already programmed into the vanilla game. Why? Because I almost guarantee that the well-meaning (and talented!) composers and developers who would do all this wonderful work have no idea about concepts such as multi-band compression, and peak-limiting, and things that literally make or break your end product. Imagine having to turn the volume down and up with each new system you fly into, or each base you land on, sometimes even having to turn your speakers off entirely, because the well-meaning composers/programmers didn't know how to limit certain frequencies from coming through, and now our cheapo computer speakers have been blown.
That's just one of MANY examples I could list. I could then go on about how music and sound effects contribute SOOOO much to the ambience of the game, and knowing that the majority of the people who would be involved in this are probably in their 20s or younger, and that demographic tends to love hiphop and electronica and drum n bass and things like that, it's inevitable that THAT is what would be programmed in, and you will lose players. I would probably be one of them. Not because I don't like drum n bass, but because there is a reason why people spend 1000's of dollars per day recording, mixing, remixing, and mastering this type of music. Sorry guys, but Garageband just won't cut it. You're gonna need a Pro Tools HD2 or HD3 rig. Got about $30,000 to drop for this project? That's just the cost of the software and DSP hardware. That doesn't include what a pro engineer is gonna charge you.
Please don't do it. It's a nice idea, it's just not practical for a game that's already been developed and is just being "modified." Save this idea for a completely brand new game that has been created from scratch.
