When Hermione punches Malfoy at first, when she's saying "That felt good" and Ron said "Not good brilliant," you could hear malfoy saying at the same time in the background "don't tell anyone you hear, I'll get that jumped up mudblood, mark my words." Then when Harry and Hermione go back in time, and they hide when Malfoy is coming, you hear him say that again, and after he finished saying it, that's when you heard Hermione and Ron start talking.