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In the past two weeks there have been several times when I have tried to adjust volume and / or keyboard brightness on my Macs on both OS X Snow Leopard and Lion only to be shown an icon with a "no" symbol on it.

Sometimes after waiting a bit the symbol will go away and these things start to work normal again.

What does this mean, what causes it, and how can I fix it?

enter image description here

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I've had this happen on my imac as well, however I have only noticed it on the volume controls. Unplugging my headphones and then plugging them back in seemed to fix it. –  Michael Dillon Jan 19 '12 at 16:13
    
Do you have an external screen connected with its own brightness controls? I have had this happen with a DisplayPort screen with speakers, and volume controls not working. I had to use those of the screen. –  Daniel Beck Jan 22 '12 at 20:41
    
while occasionally i do connect an external monitor it is never the only display and i'm pretty sure this happened when it was not connected, but perhaps it is related. –  cwd Jan 23 '12 at 0:09

2 Answers 2

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The keyboard backlight “no” seems to occur when the environment is sufficiently bright that the keyboard backlight would not be visible.

The volume “no” occurs when the audio output device is not the built-in analog output (i.e. doesn't have a local volume control); if you have no other audio devices then it's probably optical (digital) audio mode confusion as Damien Pollet's answer described. You can check the current output device(s) in the Sound system preferences pane.

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thanks. i think one more thing that causes this with the keyboard is perhaps when the battery is at less than 15%... –  cwd Jan 30 '12 at 0:15

I've noticed that when this happens, the audio ports glow red from inside. This means the mac has become confused about what is plugged in the port. They are both an electric mini-jack, and a fiber-optic audio port, and since fiber-optic is digital, I suppose volume control has to be done on the hifi equipment.

Just plug/remove and gently fiddle with the jack, and usually it comes back. It really feels like the jack is sensitive to dust or how fast/straight you insert the jack, for detecting what's plugged in…

Hope that helps

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