I turned on my new mac pro this morning and there is no sound from it. I'm only using the internal speaker at the moment. When I press the sound keys on the keyboard it make no difference. the sound display bar comes up on screen but there is a circle with a line through it below the sound level bar. Am I doing something wrong. In sound preferences I cannot click on the sound control to adjust it, same with the sound controls on the menu bar. They are greyed out (but set at top volume). Any help appreciated guys!