I've tried altering (silencing via sound recorder in accessories) 2 wav sound files (a very common looping sound file that is rather uncessessary sound-cue wise) so that I could play with windows media player playing music in the background, while still hearing the rest of the games audio.
 
Problem is, the game doesn't load after doing this, so it seems that it checks for any changes, or perhaps just can't handle them. I've tried silencing both the 11 and 22k versions of these 2 sounds but that didn't work either.
 
Now, this of course isn't a big deal, but if you happen to have an idea, or just would like to help out for the sake of a challenge, feel free. Fast music would go so well with this game with the main engine sound loop gone.
 
The wav files appear to be all of the games sound effects (short of music) and are in there own wav directory.