Unable to modify the volume with the keyboard

This icon appears when the external device or application has taken over and is to be used to adjust volume.

That also means the internal speakers are disabled.

To verify, go to system preferences sound, and look in the output tab which device is to be used to adjust volume.

See following example

If I select the Sharp TV I will get the icon like you have if I try to adjust volume on may mac, so i have to use the TV volume adjustment.

However if i select Internal speakers the volume adjustments works on my mac.

What worked for me: open up a Terminal window and run:

sudo killall coreaudiod

You may also need to run the following two commands right after:

sudo kextunload /System/Library/Extensions/AppleHDA.kext 
sudo kextload /System/Library/Extensions/AppleHDA.kext

Simple Hack: Plug in a pair of headphones, then unplug them. This will allow you to control the volume again.

I think I found a quicker and easier fix. At least it works for me.

When the volume keys don't respond (happens pretty often and I haven't figured out why) I simply put the computer in sleep mode then re-awaken it. Takes a few seconds but it works every time.