Is it only flash that you are having a problem with? MPlayer sound works? I swear I am losing my mind because I have no idea why I asked you about mplayer and what desktop you use when you only seem to be having flash audio problems. :)
It's interesting that a coworker complained about his sound not working on his FC6 + Firefox installation just today when we he tried to watch those Novell Linux Mac PC spoof ads (
http://www.novell.com/video/). What version of the flash player does it say you have when you do an "about:plugins" in the URL bar? I had no problem playing them on my FC6 + Firefox + Shockwave Flash 9.0 r31 plugin. I'll see if I can get him working tomorrow and let you know what I come up with. I am currently running Firefox version 1.5.0.10, how about you?