- what about windows mute function? or the windows volume?
- what about the codec you use (if it was winamp: check preferences -> output)
- Where you trying to hear audio cds? - If yes: probably you forgot to attach the cd-rom - to - soundcard(on mobo) - cable
- Check the devices Manager if you have a conflict with the card or if it is probably deactivated...
- Load some new drivers from the web
Hope that one of these works