The other day I had a weird problem with my Windows 7 Media Centre PC. The audio worked fine for all WMV and AVI (divx) files I had, but when I tried to play any WTV files, there was no audio.
The video played OK, just no audio. Files I had recorded on that computer, files I had watched a few days ago with audio, no longer had any sound.
After a little bit of searching on the web I thought it might be a driver problem.
So I uninstalled the drivers for the two sound devices, and then rebooted.
Upon reboot it detected the two devices again, reinstalled the drivers.
Then everything was working again. Sound on all WTV files.