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I have an external sound card (Line6 Studio GX) connected through USB. I have selected this device as the output device in sound settings:
sound settings

Now, when I press Mute (F10), the mute overlay is shown:

mute overlay

However, the sound continuous to play. The 'mute' checkbox is also not checked in Sound settings. When I check 'mute' through the settings, the sound is muted. Mute key does work for internal speakers. How can I restore the mute key for this external device?

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When you output sound through another device, it disables the volume control on the mac entirely - which is a pain, but unavoidable. You'll get the same response from your mac if you try volume up or volume down, unfortunately. –  George Pearce Dec 28 '11 at 23:05
    
Volume up/down keys work as expected on the external output device, only mute doesn't. –  bouke Dec 28 '11 at 23:30
    
Oh, my bad - assumed it was like my screen and disabled all volume controls entirely. –  George Pearce Dec 28 '11 at 23:51
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2 Answers

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My guess is that the driver for your sound device may or may not implement all the expected sound controls on the keyboard. That is to say that they are implemented correctly by the system drivers (speakers, headphone socket) but these are not in the core sound processes - hence you need your driver to support them.

I have a Fiio USB headphone amplifier that I run from my MBP, and neither the mute nor the volume controls work on the keyboard. I suspect volume is applied by analogue circuits, not digital ones - so this would make sense. But I'm a touch surprised my mute doesn't work; maybe that is expected to be implemented by the same thing that controls the volume. (Edit: my device didn't come with a driver, but was recognised as a USB sound device automatically).

Long story short - I would guess that if there is a specific driver for your device, that installing that - or finding the control panel - might help. If you don't have a driver file, try googling for one (and obviously, only download from sources you trust).

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Line6 released new drivers and the problem has disappeared. –  bouke Jan 4 '13 at 12:48
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Its likely the way line6 hooked up the usb controls through the driver as @halfer explained. they may have rushed the mac version of the driver or not had the dev resources to fully implement all of the features so it was skipped.

however, you may have luck using an application such as usb overdrive. its meant for mice, keyboards and joysticks, but it allows you to map usb connected to specific keystrokes and system controls like volume or mute.

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Can you elaborate on the part of USB overdrive? I would like to mute the line6 when pressing mute on my keyboard (when it's connected). The mute key should keep muting internal speakers if the line6 is not connected. –  bouke Jan 13 '12 at 9:41
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