I'm pretty certain that I had this after upgrading my main PC to Fedora 8, but I hit it again this weekend on my laptop: no sound playing flash videos in Firefox.
I spent quite a while piddling around with settings, downloading flash videos, re-installing stuff and googling around the problem until I finally settled upon the solution. It's quite commonly reported, once you know what to search for, and I'm fairly certain I've had and solved the self same issue before.
So, just to add to the noise, and hopefully let anyone rummaging the Net for a solution, I'll repeat it here:
You just need to ensure you install libflashsupport using your favourite package manager (I use smart).