There are plenty of posts online about issues with sound when trying to play Flash video on YouTube with Firefox. I encountered this issue today on a fresh install of Ubuntu 10.4 Lucid and managed to fix it without too much difficulty.
I had just installed Ubuntu but had not bothered to run Update Manager yet – so my first piece of advice is to update your system and reboot (if it is not already up-to-date).
I then installed the Flash-Aid Add-on in Firefox. Flash-Aid allows you to uninstall and install various instances of Flash (or Linux equivalents) via a graphical interface. Click Tools, Add-ons, Get Add-ons and then search for Flash Aid and install the Add-on.
Restart Firefox and then click Tools and Add-ons again. Select the Flash-Aid add-on and then click the Preferences button.
I decided to uninstall Flash completely and start over – so I clicked the Expert Mode check-box at the bottom of the Flash Aid window and then clicked the Removal Options tab.
I selected everything that I had possibly installed previously and then clicked the Installation Options tab. Here I selected Adobe, from repositories.
Then I simply clicked the Script Preview tab followed by the Execute button – this ran the uninstall scripts followed by the installation script.
After this I still did not have sound working on YouTube – but running Update Manager and rebooting the computer fixed the issue.
Flash-Aid is a great tool for quickly and easily managing the version of Flash that you wish to use.