Okay, annoying as hell thing happened a couple of days ago: sounds stopped playing in Firefox on my Mac.

Apparently (according to macosxhints), if you can’t hear anything in Firefox for the Mac, the most likely culprit is a program having changed the sample rate setting.

To fix, go to Finder -> Applications -> Utilities -> Audio MIDI Settings and change Audio Output Format 44100.0Hz and then you will have sound again!