Speaker test good but no sound

Hello, I have a Sony Vio running Vista and I have been having trouble with the sound. I go to Control Panel\Hardware and Sound\Manage Auido Devices\ and test the speakers and I hear a beep in each speaker so they are working BUT if I go to lets say YouTube or something I get no sound threw the speakers. Sometimes if I restart the laptop the sound works fine. Any sugestions?

Thanks,
Chad