[CentOS] Just can't get sound working in Firefox

I have been all up and down my Firefox options, and all up and down
whatever sound card configurations I could find in Gnome. And I just
can't get sound to work. Whether it's a Flash animation, a Quicktime
movie, or anything, they all run, but no sound.

All other applications that run sound, like Xine or XMMS, work fine
(although mysteriously XMMS every now and again loses it's connection to
my sound device.)

When I look on Google for help, every FireFox related page ultimately
says "check your sound configuration" without much more to offer.

I'm totally stumped. I'm not even sure where to look anymore.

Any suggestions?

Dave