If the above suggestions don't work...
Try reseating the card in the PCI slot. You can also try a different slot.
If you have access to more than one computer, try the sound card in a different machine.
Double check DirectX and make sure there are no errors there.
Go to the sound card manufacturer's site and ensure you have the latest drivers installed.
Hit it with a 10lb sledge hammer... You'll have no problem with this after going through all the above with no positive results. Might not fix the sound, but trust me, you'll feel better! 
EDIT: If your sound is part of the motherboard, disregard the first two suggestions...