One day I had sound in YouTube videos, the next I didn’t. Doug’s blog to the rescue. Basically:
sudo apt-get install alsa-oss
edit /etc/firefox/firefoxrc and ensure that you have “FIREFOX_DSP=aoss”
restart all Firefoxes
and you should be right.
I don’t think the Dapper team have quite got the hang of not breaking things that worked previously on system upgrade. My advice is – if you have a Breezy box working perfectly, stick with it. If it’s a new box, Dapper is fine.