Rich,
I couldn't agree more with the need to have a one click way of turning voice commands on and off. My Tom Tom Via does this perfectly.
I have resigned myself to using the navigation system without voice commands. - At least the screen splits into two on the approach to turn instructions - which does give a visual reminder.
Using the most appropriate map scaling for the journey is a big help and generally on cross country trips the no sound option is OK - if the journey includes a tricky city trip I use the Tom Tom.
I can't see why a firmware update can't be produced which would allow, say, a two click operation to mute / unmute.
Loving the CT though!!