What Causes This “No Audio" Problem?
There are various reasons why you might experience problems with a lack of audio in YouTube or other rich media websites viewed in Firefox on Ubuntu, and these all relate to the version of Adobe Flash Player that is in use.
Introduced in 1996, Flash Player has become a must-have plugin for all pre-HTML5 web browsers, enabling websites to easily stream media content and games to users’ computers. Its position as the key technology in this area is under threat, however, with the arrival of HTML5, Microsoft’s own Silverlight technology and Apple’s refusal to allow the technology on any of the iOS devices, not to mention delays introducing a 64-bit version.
When an Ubuntu computer is upgraded from, say, Ubuntu 8.04 to Ubuntu 10.10, the issue with Flash Player not playing sound in Firefox occurs. Fortunately, this is a problem that can be resolved using various methods, such as altering sound settings or upgrading Flash.
Before embarking on either of these fixes, however, spend a few minutes with your computer hardware, checking that no headphones are plugged in or that any external speakers that you use are powered on. Basically, you want to rule out any other aspects beyond these two.