I'm assuming you've had a rummage around in System Preferences>Sound>Output...
With that symbol showing up, it indicates that there is no sound output device even connected (including the built-in speakers). If there is no red light in the headphone port (which you can definitely see -- even in a well-lit room -- if you are staring directly into the port) then it is most likely a logic board issue.
I'd still try the ol' 3.5mm jack in-out trick to try and trigger the optical out to turn off again. Whether he's seeing the light or not these are the exact symptoms of the stuck switch. It's worth a try.
Well, when you see the red light, that does indicate a stuck switch, but when there is no red light, there's a chance that the logic board is the issue, as the port is connected to the logic board. I had these same symptoms and had to have my logic board replaced in order to fix the issue.
Really looks like a classical problem with the audio jack. Are you absolutely sure there isn't a red light in there? It is not very powerful, so look at it in a darkened room.
I'd suggest making a Genius Bar appointment. They will figure out what's wrong and most likely get your logic board replaced.
You can read my thread about the same issues happening to me, although mine was a bit worse as my speakers would work, but with loud popping/crackling sounds that made them unusable.
Here's the link:
CLICK HERE.
I documented everything just in case other people had similar issues like I did and so they would know what to do, so hopefully my thread will help, but either way, definitely make a Genius Bar appointment.
