Ah- Hah !!
You've lost the drivers or your sound card is bad, but most probably its the sound drivers. Happened to me once, & I took it to my local geeks (not the ones riding the VW's) and they gave it back to me with a CD that had card & drivers on it (in case it happened again)- I still have it. Plain orange cardboard CD envelope.
It was called :
2000 ASOUND Series Ver.3.0
& had on it:
ASound Express -PCI Sound Card
ASOUND Gold - ISA Sound card
of course it cost me $59.95 to get it.
(Basic minimum for my geeks to work on a 'puter)