Most video websites have Flash as there default player. If Adobe Flash Player / browser plug-in does not work you really don't have much of a choose on what to watch on-line. A lot of people are having problem with their Flash videos not having sound from what I saw in my search. I tried most things that where written online but nothing worked. Granted most were written towards Windows but not all.

First of all I want to make it clear that my sound card was working so if your sound card is not working you have a totally different problem then mine. Make sure Linux recognizes your sound card. Next go to:
Sound Mixer.
Raise the volume on: PCM (Pulse-code modulation)
and that is it.
Sometimes the hardest things are fixed the simplest ways.

This worked for me I do not guarantee it will work for you. I tried everything else and this was the only thing that worked for me. Hope this helps.