|Maybe here? >>> http://www.via.com.tw/en/support/dr...|
Have you made any changes that may have affected the audio? If you're running XP, you must have SP3 installed for HD audio. If not, there's a special Universal Audio Architecture (UAA) patch that has to be installed before installing the actual audio drivers.
Also, I see you mentioned nvidia in the title, is that your graphics card? Does it have an HDMI port? It's possible the nvidia HDMI sound took over as the default audio when the nvidia drivers were installed. Did you look in Device Manager to see if both audio drivers are listed & there's no errors or warnings?