i have tried pushing f4 nothing, I have a red light showing on the mic input on the side of my mac,?
what do I do if my mac has no sound?
Poke it with a stick 
.
No, seriously! The connection points in the headphone port are sticking causing the "optical output" setting to engage. If this is the problem, there will be a red light glowing in the port.
Plug in and unplug the headphones a few times to free them up.
If that doesn't help, some people have had success with a toothpick ... extremely gently, of course.
See:
http://discussions.apple.com/thread.jspa?messageID=6720744�
Hope this helps.
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