IMac :: Restriction Symbol In Sound Box

Occasionally I will turn on my IMac and I wont have any sound. When I try to turn up or down the sound, there is a restriction symbol in the sound box that pops up and nothing will happen. The old restart usually works. However, when I try to plug in headphones or an external audio cable, nothing changes. I still get sound out of the computer, and it works like there is nothing in the jack. All software is up to date, I reset PRAM and did the "unplug everything" drill. Still no change. When I did the unplug everything, its almost notorious for having the first problem of "no sound".