Windows 7: Muted Computer!!? No sound - device/driver problem?

Hello intelligent internet folk,



Today my sturdy Sony Viao turned mute.
The sound was working, well and true up to the point when it just stopped.
The computer seems to think that the audio is working, showing movement in the speaker thing on the control panel, but there is no sound even if headphones/speakers are plugged in.
I have tried disabling/enabling the audio device, and turning the computer on/off.
There are no start-up sounds. No sound at all, and as far as I can tell, the settings all seem the same.
I'm at a loss as to how to fix this most irritating problem.
Do I need to install a new 'driver?
please help!
I can't go on much longer with no sound.

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