Hi, Russell barely EVER barks either. I have read that you click and treat when he barks, then gradually introduce the command speak (free shaping 'speak') then click and treat for "quiet" when he stops barking, gradually adding the command (free shaping 'quiet'). The only tutorials I have found is for persistant barkers to lessen their barking by putting it on command so you can control it. The only thing that I can think is that it may be very slow for Murphy and Russ, as the chance to click and treat are not many. If he does get the hint that barking is good then it would increase, then you do the above. I'm sure most people would say that we should be pleased to have quiet dogs! Let me know if you find anything else!!
To get my two to bark so I could put it on command we did play sessions in the garden A bit of shouting, dancing, playing chase and mucking about acting like a loon soon got them excited enough to start barking. Try it maybe?! If nothing else you'll give them a good run!