Going on interactive experience and observations of live animals, I'd be more inclined to subscribe to the visual theory. Even a dog that was raised with no contact to other dogs will growl when faced with something scary, odd or intimidating, much as a happy or aroused cat is bound to purr and roflcopter all over the floor. There is no such thing as proper 'language', just aural signals that are closer to simplified icons than language.
If you're lucky enough to witness a sleeping dog dream, it mostly seems to center on running and eating, in whatever context happens to happen in that sweet little brain. Depending on the individual dog being observed, that might or might not involve more vocal ways of handling life, reality and the experiences you throw at your dog.
An isolated dog will not make much noise. All noises it makes are made with the intent of communication with other dogs or, which is rather amazing, humans. That's a perk that allows dogs to do jobs like herding, guarding or otherwise try and influence other creatures, no matter the species.
Maybe you are lucky enough to witness a dog herding sheep or, more impressive, angry cows and bulls. A dog with a job that takes its job seriously will use noise to make a point, but that tends to be a last resort or a sign of failure - it's really all about spatial awareness and achieving more or less total control over the dance of trigger-response behaviour. Every move of a troublesome cattle individual will trigger a motion response by the dog. As soon as the dog first gets the general idea of how things work, the cattle just won't stand a chance in the brainpower department. A cow is bound to judge your size at more or less two-dimensional face value, a dog will not fall for such tricks and be more WTF? about it.
Our last dog was a Rott that was very vocal when around humans. Absolutely silent when in herding or hunting mode, as his physical presence just did the trick, no matter if it was sheep, geese or Angus cattle. As a baby, he'd want to bark a lot, and it took just a couple of weeks to lovingly shush him to make him understand that indoor barking is frowned upon. He developed a silent bark that worked like a charm and confused the hell out of people that haven't seen that before. He still barked, going through all the motions, but not using his lovely baritone voice. He only got loud to shut up other dogs that would not otherwise shut up. Everyone was cool with that.