I've been helping my friend with his Sony Vaio desktop with Windows XP.
The computer was heavily infected with no firewall, or antivirus.
We cleaned everything out, turned Windows firewall on, installed AVG.
All said, and done, there is no SOUND, which worked before.
This is what we tried:
- uninstall/reinstall on-board sound drivers
- he bought new sound card, we disabled on-board sound, installed new card - at this point, he is able to play music CD, music from flash drive, but there is no system (Windows) sound, nothing will play from inside Windows (including browsers)
- sfc finds nothing
- Windows repair install does nothing

Any other ideas, before going for reformat, reinstall?volume settings? Mute box checked?
I had similiar problem and after lots of FRUSTRATION found that under device manager, sound, video & game controllers, PCI Audio Device the Device Usage was disabled.All sound settings (volume, mute, etc.) checked out.
No errors in Control Panel.SOLVED: installing K-Lite codec pack, fixed the problem Congratulations, knew you would get it. Thanks for posting solution. Man, It's been bugging me last two days.
I HATE sound problems, because what worked out this time, not necessary works in NEXT case...grrrrrrrrrrrr.Sound issues are indeed aggravating....have been since the DOS gaming days.
All you have to misss is one thing and WHAM.