Apple Keyboard volume keys not working

Recently, my Apple Keyboard (the original model that takes AA batteries) has stopped control of speaker volume using the F11 and F12 keys. These two keys now bring up a warning image showing what I assume means "no speaker." I think this situation has occurred as a result of the last iTunes upgrade but I'm not entirely sure of the exact date or change to iTunes or other OS software. See inserted screenshot of warning against background of iTunes.

I'm running macOS Sierra 10.12.1 and iTunes 12.5.17.3 on a Mac mini.

Any suggestions?