no sound playing on flash based web content?

Hi, I have a problem with my browser, there is no sound playing on any flash based web content such as

- Youtube Videos

- Flash Games

- Etc...

Though for java applets/games (Runescape) the sound is working fine.

And the rest of the sound on my pc is working ok, i can play music and games fine, and my sound works on msn/webcam.

I've checked the settings in the control panel, none of them are muted and they're all turned up.

I've downloaded and made sure flash and the required plug-ins for my browser are upto date.

Any help would be appreciated (my browser is firefox, and I dont have IE installed on my computer to test in there)

Thanks