I'm assuming your Winamp internal volume (the volume control on Winamp itself) is at 100% when you say your MP3s are too quiet. (Otherwise, it'd just be a matter of turning Winamp's volume up, as it works independently, within the range of what your WAV mixer is set to.) If it is maxed out and some MP3s are still to quiet, I think you'd be best served by normalizing and reencoding those problem tracks. If *all* of your music was too loud even with Winamp at full volume, I guess there are DSP plugins that could apply gain, but I haven't researched that, and it might be subject to some distortion, not to mention blowing your ears off if you encounter a properly normalized track.

Strange situation, those "work sounds" must be really loud if they can't compete with Winamp at 100%.