For all you Scooby fans out here (and there), most of the time he enunciates most consonants and the like with an "r" sound (to emulate typical dog talk). But as the series wore on, he seemed to pronounce certain consonants correctly. This was evident in one episode of 1983's The New Scooby and Scrappy-Doo Show, titled "Wizards and Warlocks." In one scene of this episode, there is a floor with the words "RHIRD REVEL - ROTS OF RUCK" - in traditional Scooby talk. But Scooby pronounced these words in normal human talk, saying, of course, "THIRD LEVEL - LOTS OF LUCK."
~Ben