So he was making some noise before, and now he's quieter? He is probably confused about what he's supposed to do. He may also be positioning his body a certain way, or something.
You should be able to build the behavior back up; reward EVERY noise, with bigger treats on louder ones. He has to be really confident on the command before you can start fine-tuning it. Also, I don't know if you are, but you can't correct him for barking at other times, because that will confuse him.
Is there anything that always gets a big ol' bark out of him? The doorbell maybe? You can also make the signal and say "Speak" whenever he barks, and treat him, so the association will strengthen.
Let me know if you have any luck.
