Why is my MacBook Pro stuck on mute?

The headphone jack had a red LED glowing inside, which clued me in that it thought there was an optical cable plugged in. Plugging and unplugging a pair of headphones resolved the problem.

I had same problem, but none of these suggestions helped.

I was almost desperate, since I found this interesting article: How to reset the audio system in OS X

So I begun to follow the steps suggested into the article. But I had to do a little more in order to restore audio capability. Here the steps I did:

$ ps -ef | grep audio
202   376     1   0 10:01AM ??         0:00.86 /usr/sbin/coreaudiod
202   379     1   0 10:01AM ??         0:00.04 com.apple.audio.DriverHelper
501   475     1   0 10:01AM ??         0:00.02 com.apple.audio.SandboxHelper
501   528   504   0 10:02AM ttys000    0:00.00 grep audio

$ sudo killall audio.DriverHelper
$ sudo killall audio.SandboxHelper
$ sudo killall coreaudiod

then open "Audio MIDI Setup" utility.

And then toggling settings such as these can reinitialize the audio driver.

Maybe this procedure will rewrite the audio configuration