Ubuntu: no sound in Flash
One of the most common problems with a fresh Ubuntu installation is sound not working, particularly with Flash videos. This is apparently due to a bad configuration of PulseAudio. The web is riddled with blog entries and forum posts about how to fix this for various versions of Ubuntu.
The following is the most dependable and up-to-date guide I have found. It has information for Ubuntu 8.04 (Hardy), 8.10 (Intrepid), and 9.04 (Jaunty).
http://ubuntuforums.org/showthread.php?t=789578