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I plug my machine into a projector/sound amplification setup regularly for teaching. Often, when I get back home afterwards, the sound volume controls no longer work. Neither the dedicated volume keys in the function key row, nor the volume slider in the menu bar have any affect on output volume. Also, the keys changes don't affect the slider position, or vice versa.

I've done some searching and testing and the only working solution I've found is to reboot. Not a great solution. I've tried switching output devices (no change) and restarting coreaudio (no change). I'm on 10.6.8.

Any non-reboot suggestions?

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mine never shows the prohibitory sign. it acts like it's changing the volume as requested, but those changes have no aural effect. – mix Sep 3 '12 at 3:33
Can you change the volume in System Preferences? – user495470 Sep 3 '12 at 5:30
If you take a look at your System Preferences < Sound, what do you see on "output"? I think the projector will be listed there and you Mac is still thinking you're connected to the projector. – Michiel Sep 3 '12 at 6:06
@Lri yes i can change the volume successfully in the sound pref panel, although changes in the slider there do not affect the slider in the menu bar, and after adjusting vol there the keys still don't work – mix Sep 3 '12 at 6:28

According to this thread, holding down any of the volume keys as you plug or unplug the audio jack will solve this problem. It's a known issue.

See this thread:

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I have had that problem with my Mac book. The sound was not working after plugging my computer in a pa system. After an hour of searching I found the solution. Plug in your headphone into the port your sound slider should work and you have sound coming through your headphone, reboot your computer keeping your headphone plugin and remove/unplug your headphone your sound slider should work again. The Mac helpline didn't provide a solution except your Mac needs to be serviced! Hope it helps.

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