"Problem solved. I installed the drivers for my graphics card and that also installed the correct audio drivers.
Thanks all for your help."
Wow. That is a most unusual fix! Very counterintuitive.
This caught my eye because I had the exact same problem. I was unable to locate or reinstall the drivers for my mickey mouse onboard sound device from my local computer fixit shop about 4 years earlier, so I ran out and bought the Soundblaster Audigy from Creative Labs that I'd always wanted anyway. Running to Office Depot and back and installing the new sound card took under an hour. Now the intro to Return to Castle Wolfenstein runs properly for the first time without the sound fizzling out after 20 seconds.