Why is my MacBook Pro stuck on mute? I cannot hear any audio from my MacBook Pro, running OS X 10.7. Pressing F10/F11/F12 shows the volume overlay, but it stays at max volume with a (/) icon below it. In the Sound control panel or from the sound icon in the menu bar I can adjust the volume, but the Mute checkbox is greyed out. Rebooting does not resolve the problem.
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.