I have a Mac mini and the sound has disappeared. I have also lost the sound icon off the toolbar. I went to preferences for the sound and see that mute is checked. I can't un-check it because it is in gray-scale. Any ideas?
I had it once before. You should check your audio-input. Good chances there is a red light inside your input.
Inside your audio jack, there is some kind of spring which controls your output (internal speakers or headphones). And for some reason, this switch inside the audio jack is stuck. Try using a toothpick (if you dare) and try to unlock the quill.
If using a toothpick did not work, you need to take the computer in for service, because there is probably something wrong with your I/O board.
If there is no red-light, my answer expires.
About the disappearing of the icon in the menu-bar, you should try and reset your PRAM.