On page 169 of my copy of Harry Potter and the prisoner of Azkaban Harry is called into Professor Lupin's offices (this is during the day that everyone's off to Hogsmeade, and Harry has to stay behind. Chapter eight). After a while professor Lupin says the name of Voldemort, and then J.K. Rowling says this: "Harry stared. Not only was this the last awnser he'd expected, but Lupin had said Voldemorts name. The only person Harry had ever heard say the name aloud (apart from himself) was professor Dumbledore." The thing is that in the first book (Harry potter and the philosopher's stone) Hagrid told Harry about what really happened to his parents on page 45 (Chapter 4). Here J.K. Rowling says this: "'could you write it down?' harry suggested. 'nah can't spell it. All right - Voldemort.'" So Harry didn't only hear Dumbledore say Voldemort's name, but also heard Hagrid say it.