I'm not positive, but if you have drivers for that sound card such to emulate a SB16 (which most sound cards should have software to emulate these days), then you should be able to use the SB16 setting in MPXPlay.
Did you try any of the setting for MPXPlay, other than the (default) autodetect?
I bought a motherboard with only a sound card built in, a Crystal chipset, works perfectly with MPXPlay. Do you know what chipset you have for your soundcard?
Don't give up! DOS w/ MPXPlay and MPXF (front end) are by far the best for doing just the simple task of playing mp3s ... fastest boot, unbelievably stable, won't moan and complain when you shut it down ... ahh the list can go on.