MacBook Pro Sound

OK - my MacBook Pro sound/volume wokrs upon restarting. After a few minutes there will be NO sound when I'm on the internet (whether it's thru Safari or Firefox) BUT there will still be sound when I send an email and if I shut down, the chime is there, too. I've hit the F10, F11, F12 buttons, raised the volume control through system preferences and the icon on the top of the computer. Does anyone have any ideas?? It's so frustrating - especially since the volume/sounds work fine if I restart (but never lasts more than a few minutes).

Thanks!