I'm searching for something similar. I'd like a way to play MP3's behind a game, and have control over the volume of the Mame game playing and the MP3 player, essentially allowing Mame volume to slide down while MP3 volume slides up. That way I can still hear the game softly for audio clues while jaming out to the the current track. Idealy, this software could be tied to command or hotkeys.
Please post any additional findings.
-csa