Plug and play software device enumerator and how to reinstall it (XP)
This often fixes audio problems. If, under system in device manager, you don't see the device listed then you can install it by:
copy c:\windows\inf\machine.inf to a temp location.
Remove the line in the temp copy that says "ExcludeFromSelect=*"
Go to add new hardware and get to the 'all hardware' point, choose have disk and point to that temp location.
Scroll down in the resulting list and choose the device. After installing, uninstall the soundcard/etc and rescan for hardware changes (right click the root node of the device manager listing and choose rescan... never reboot or use add new hardware wizard for driver changes as that's unnecessary and slow)