Hi. Sometimes, even if your computer's volume is set at maximum, the volume of a particular application (such as Firefox) might be muted. Please check the What to do if Firefox won't play any sounds article. I hope that helps :D

I get audio and video at Al Jazeera and BBC websites. At YouTube I only get video.

The Adobe website http://www.adobe.com/software/flash/about/ indicates that I "... have version 10,3,181,34 installed"

If this is so specific to Youtube, I'm sorry, but I have to ask, have you checked the Youtube player's volume?