Recently I got an upgrade for my Ubuntu 8.10 (Intrepid Ibex) and it broke the Firefox Flash sound. My wife was mad at me because now we could not watch the movie that we had decided to watch, so I had to do something real quick.
Here is what solved the problem for me in a matter of 30 minutes, so, not that quick but at least we did watch the movie
Goto Administration –> Synaptic Package Manager –> <Search for flash nonfree>
This should show you the following screen
Mark the flashplugin-nonfree-extrasound for installation and you would be good to go and watch a movie. Enjoy!