I have just noticed this today and I'm not aware Firefox has updated recently or that I have changed any settings but I no longer get any audio track on You Tube videos. The small speaker icon has an "X" next to it and clicking it has no effect. I have the latest version of Flash Player installed.
I've been watching a lot of BBCs iPlayer and the ITV Player recently... I wonder if one of those has changed something?
If anyone else has had this problem and found out how to fix it I'd be very grateful for your help!
EDIT: BTW this seems to only affect You Tube, other streaming video sites like Dailymotion are fine.