Fedora 12 Firefox 64 bit flash pulse audio no sound - resolved
The situation was that I had 2 Fedora 12 boxen.
One played flash videos with sound, and one did not!
Eventually I found the actual problem, and it was trivially fixed (I don't want to remember the many blind alleys I went into first...).
When I looked at
System ==> Sound Preferences ==> Sound
and selected the 'Applications' tab, I got the line:
"No application is currently running or recording audio.",
but the Mute check box to the right of the 'Output Volume' slider at the top of the tab was unticked.
However, when I was running a flash video, I had a line:
"ALSA plug-in (npviewer.bin) ..." with another Mute check box on the right that was ticked! Unticking this new Mute check box, enabled Firefox to play YouTube video's with sound!