Audio problem: When Hedwig flies from Harry's arm, or owls deliver mail, you hear the sound of wingbeats. However, owl's feathers are specially adapted to allow for completely silent flight. See here for an explanation. http://www.owlpages.com/articles.php?section=Owl+Physiology&title=Feathers.
Audio problem: In the scene at the very end of the movie when Hermione and Harry are getting on the train, Hermione says, "Feels a bit strange going home, doesn't it?" Harry responds, "I'm not going home, not really." If you look closely, Harry says the words and his mouth moves. Then the camera changes angles and Harry mouths the same words once more, but no sound comes out.