This last week I upgrade a box at home and all my sound through Firefox was gone… no more flash movies with sound here… luckily the fix is easy:
1. sudo [your-favorite-editor-here] /etc/firefox/firefoxrc
2. change line FIREFOX_DSP=”none” to FIREFOX_DSP=”auto”
3. done!
Restart firefox and if you flash player is working, you should have sound! ;-)