Both my two 'speak' on command.
I just made treats for them held the treat so they cold see it and said 'speak' - as soon as they barked they got a treat!
I did pretty much the same as Lucy. But when that didnt work, I put Betty in a sit, and kept her attention with a treat, then I said the word 'speak' and barked myself
I know I sound krackers, but after a few barks, Betty started woofing back.
Just kept saying 'speak' while she was doing it along with lots of fuss and treating