I had a similar problem with a client's PC - no sound with GoogleVideo or YouTube but Windows Media Player and QuickTime worked fine. I tried the wavemapper registry thing and it didn't make any difference. Uninstalling and reinstalling Flash (with a reboot inbetween) and using the Flash uninstall tool and the stand-alone Flash installer didn't work either.

After trying everything I could think of, I finally realised that whenever I rebooted the computer, I could hear the "Start Windows" startup music but it seemed to have the last note or two clipped off. So I went to Start > Control Panel > Sounds and Audio Devices > Sound tab and tried playing some of the sounds in the "Program events" box and NONE of them worked (not even the "Start Windows" startup music!). I noticed at this point that there was nothing in the "Sound scheme" box either - I tried changing it to Windows Default but still no difference to the sounds.

Finally I reasoned that if the Windows default sounds weren't working, then this had to be more than just a problem with Flash (ie, a problem with the Windows installation) so I downloaded the sound card driver for this PC's motherboard (if you've got the original motherboard driver disc then you're going to be in luck), uninstalled the sound driver that was already installed via Add/Remove programs, rebooted the computer and installed the new driver and hey presto!, the sound worked for EVERYTHING!