Hopefully this support article is what you need:

http://support.mozilla.com/en-US/kb/No+sound+in+Firefox

I have read all that and done the steps and I still can't hear any sound.

Clear the Flash cookies and settings.

Flash Website Storage Settings panel:

http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

See also Flash Local storage settings

http://www.macromedia.com/support/documentation/en/flashplayer/help/help02.html

After upgrading to 4.0b5 today, my sound is acting very strange also.

I run my audio out via the headphone jack, and now when FireFox should be playing a sound - suddenly the audio is routed through the computer speakers for a second and then it switches back to the headphones.

Everything is playing through the headphones fine, but when FireFox has audio to play - it routes it through the computer speakers instead and then toggles back over to the headphones.

Note - the FIREFOX sounds do NOT play when this happens, but any audio im playing on the computer other than with FireFox DOES play when it switches back and forth between the computer speakers and the headphone jack.

As others have mentioned I also got that error message after updating that said 'Alert' and something about a process that could not be completed, or something to that effect. I searched the message earlier and it said there was an fix in the works for it already - I hope the fix for whatever that was will fix this funky audio problem.

Done this and still no sound