Having upgraded the firmware I am now finding that after several days of normal operation my GMC-320 Plus starts making ticking noises, i.e. as if the 'Speaker' option had been set to 'ON'. However, when this happens, not only can I see that it is set to 'OFF' but there is also no loudspeaker icon showing on the screen.
I have found that to reset the device it is not necessary to change the loudspeaker setting - simply to view it. To do this I simply follow this sequence: