There’s been huge issues with Flash 9 and embedded flash videos either not playing at all or playing without sound. This problem is sometimes associated with Firefox not playing video sounds, YouTube embedded videos not playing, and with Flash 9 itself.
There’s some fixes out there requireing updates to the registry, updates to the Flash player settings, and updates to your computer’s sound settings.
However, I found that none of this worked for me and I think that can be said for most everyone else for the issue. In the end, I found that you can fix the no sound in flash video problems can be fixed by ditching Flash 9 altogether and going straight to the Flash 10 Beta:
That’s it. So far I’m now getting sound and video is playing with the flash player from youtube videos, embedded videos, in both Chrome and Firefox.