Muted leopard bug?

I’m trying to play some music from iTunes, but I can’t seem to get anything to play. Space bar doesn’t work, play button doesn’t work, nothing. Then I try volume up, down keys, and controls in iTunes and the menu bar, none of which works. Then I open up the Sound preference panel, and lo and behold the mute checkbox is checked, and it’s greyed out so I can’t change its state, even if the volume up and down keys indicate that the audio is not muted!I did have similar problems with muting under Tiger, but don’t think I ever noticed that the mute checkbox was permanently checked.