If your using the built in sound card in the mobo, then you should be able to (I have the same mobo and have the same games and can play mp3 while playing) just to get that out of the way.
Can you open and mp3 in winamp and then open another mp3 in Windows Media Player and have them play at the same time?
I know in steam there is some sort of built in mp3 player (that may be linked to winamp, which could possible be stopping the mp3 from playing. If you minimize the game, can you play mp3's, does it stop the mp3's player or just cut the sound?