I bought a new 27" iMac on July 2, 2013. Latest operating system, added extra memory (32GB)

Anyway, starting about July 15 the volume control on the keyboard would freeze up. A circle with a diagonal line would appear and I couldn't control the volume up or down.

The only way I could get it back was to re-boot the computer. It would work for about 5 days and then re-occur.

I called Apple Care and we tried a System Management re-set. Of course because I re-booted the computer to do the reset, it worked, because of the restart.

It keep on re-occurring. Finally after trying many things, I happened to go to System Preferences > Sound > Output. I was using Harman/Kardon Sound Sticks which I've had for a number of years with no problem. I clicked on "Internal Speakers" and had control of the volume again. Back to SS and it was locked up with no control.

I unplugged the USB into the Sound Sticks and the control came back when using the SS for external sound.

Hope this helps anyone with a similar problem.