Ohh, I saw this post and I had to respond!
I'm an animal biology major, just 1.25 quarters away from graduation (xmas time can't come soon enough!). Anyway, from all the knowledge I've gained at my university and college, I would like to make an educated guess about the lack vocalization that bearded dragons exhibit.
I think most reptiles make little, if any noise unless they live in the tropics, where thermoregulation isn't much of an issue due to the lack of temperature fluctuation, or if the reptile has some sort of defense mechanism they want to make known (like the rattlesnake and it's rattle/venom combo). I think that beardie's don't make noise because it's energetically expensive, but more than that, they are dependent upon outside temps for most of their actions. If there was a stimulus that the dragons responded to with noise, like a mating call, they'd be sitting ducks for predators that heard that mating call, especially if they couldn't get away fast enough. They're not the fastest lizard in the outback, so it would make sense anyway that they'd want to keep quiet.
Anyway, that's just my little hypothesis!