Although I have come across this problem, I did have a quick search on google for you.
A couple of sites suggest scanning your computer for Malware for any potential infections.
Secondly, I did find one blogger who mentions he fixed his issue with a registry fix.
If you paranoid about downloads you can fix it manually, here’s how:
start regedt32 (Win+R, type “regedt32″), go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Drivers32 location and make sure you have wavemapper with value msacm32.drv
His site is here: http://sb2.info/flash-player-9-missing-sound-issue-solved/comment-page-1/
I Hope this helps.