My mute key isn't working properly

[rbf1138]

The icon comes up on the screen when I hit F10/Mute on my 2012 Air, but it doesn't silence the sound. I've used a USB DAC so that may have changed some functionality, but I've checked in my sound settings and I've set it back to the default output but it's still not working. Any ideas/fixes?

 

[MarkMS]

If you altered the settings and are sure they are right, I'd probably try a PRAM reset. Some sound settings are stored there, so it might help.

http://support.apple.com/kb/HT1379