I am going to suggest you boot into your system disk and select utilities/disk repair. Run disk repair until it reports no trouble found and let us know if that clears your problem. If not we will try something else. Ralph

Try going to the Apple in upper left of desktop screen. Then click on System Preferences. Under the heading "Hardware", click on Sound. There, make certain "Show Volume in Menu Bar" box is checked. Then click on "Output" tab at top. In Output, be sure your output device is selected in "Select a device for sound output:" Then, at the bottom, make sure the volume slider is set high enough and also be certain that the "Mute" box does NOT have a check mark in it. And once again, verify that the box next to Show volume in menu bar IS checked. Give it a whirl and see if that helps.

oops… what I have in my "Select a device for sound output:" is Headphones and the Type is Built-in Output. I use an external computer sound system.