WELL if he is quiet when you say "speak", then have "speak" be the command to make him quiet!!

Dogs don't know english and just know to associate a sound (or a word to us, since we DO speak english) with an action. lol. Just because you say "sit!" doesn't mean the dog will sit there and think "oh, sit means put my behind on the ground, ok!". I mean you could teach your dog that "sit" really means to fetch, "lay down" means to speak, "speak" means to sit, etc. They're just sounds to them