A few days ago I noticed that videos were not playing in sound in FireFox. My laptop was still playing sound ok through windows media player and internet explorer but not through FireFox. I checked my mixer panel via the sound icon and it said that sound was not working .This meant I couldn't watch videos on youtube etc.
I checked on the web and it seems that thousands of other people have the same problem. However, there seemed to be dozens of different solutions and the first few I tried didn't work.
Eventually I found a codec pack which seems to have fixed it. All you need to do is download a codec pack called codecs6030_allin1.exe, install it and then restart firefox. My laptop uses Vista so if you have a different operating system it may not work but I'd give it a try anyways.
Good luck :)