This seems to be an ongoing problem... I found that there are many claimed soultions out there, so I'd give them all a try and you might find one that works. As for me, I had no sound on any web browser, or system sounds (start-up sounds, mouse clicks) but did have sound on media player.
*First thing I did was the most obvious,
Check the sound properties and make sure nothing was muted and that the sound icon was checked and all the volume levels were up. no good.
*Second I un/reinstalled all of my flash devices. adobe, quicktime, etc.. didn't work
*Third I un/reinstalled my sound drivers, then eventually bought a new soundcard figuring maybe the integrated sound had malfunctioned, but to no avail.
*Next I tried checking the registry and seeing if wavemapper (as mentioned in other forums) was there- it was.
*Third I downloaded the K-lite codec pack, figuring the problem was in my registry. The fix all part was greyed out...
*I decided to try a system restore on a date before I lost my sound, wouldn't work.
Finally being frustrated and out of ideas I decided to pull out my OS cd and go with a Windows Repair.
It's not hard to do, and it doesn't remove any of your saved data such as songs, pictures, and word documents. But I would make a backup of any data you wouldn't want to lose, just in case. This was my last resort before a complete reformat.
Here's a good link to go by
This worked great for me, I now have sound, all of my files, and my system runs just as smooth as ever.