Audio internet sound stopped working.

Apologies if I'm retreading old ground but I can't seem to find a workable answer. The audio on my internet has suddenly stopped working. I'm using leopard 10.5.8 on an imac and using safari with no problems but now can't hear anything. It just suddenly stopped working. Audio on the rest of the mac is fine and I tried firefox but had the same problem. Followed info re: audio midi setup and initially said that audio output was not supported (changing properties to 'built in output' gave me 'audio output' options but that didn't help) . Everything is set at 441hz. Also after following some internet advice it seems that garageband has disappeared (I've never used it and don't know if this make a difference). I've tried all the solutions that I've found on the internet but none have worked for me. Any help would be greatly appreciated - I think I'm still under mac care warranty but would like to try to understand it myself.

Is there a particular website where you're experiencing the issue? Can you share an example?

Oooops.

Er.... yes, it was youtube. I'm a little embarrassed to say that I'd accidentally pressed the mute button without realising. Thanks for your reply though.