Audio problem: In the pet shop the cashier starts moving her mouth before she says anything to Melanie. (00:02:25)
Audio problem: As Melanie pulls up in front of the general store a woman with bags of groceries exits the store. They dub in the sound of the door closing but they didn't dub the sound of the bell on top of the door that we hear later as Melanie walks in. (00:12:20)
Audio problem: They use a different overdubbed tone for the bell on the door of the general store when the manager opens it. Before as Melanie walked into the store we heard the real sound. (00:13:10)
Audio problem: As Melanie and the manager leave his store to look at the Mitch's house, they overdub the bell sound when he opens it but neglect to when he closes it. (00:13:15)
Audio problem: As Melanie parks in front of Annie's house the audio of her car engine is not in sync with the film. The engine sound stops before the exhaust stops coming out of her tail pipe and she eventually pops the clutch to stop the engine from running on. (00:16:00)
Audio problem: When Annie says, "Did you drive up from San Francisco by the coast road?" her lips are mouthing something different. (00:17:55)
Audio problem: Before Melanie starts off across the bay we hear the boat keeper start the boat's engine. But this is only an over-dub, the engine's not really running as there's no turbulence behind the boat like in the next two shots. (00:20:30)
Audio problem: After leaving the card by the bird cage Melanie walks on the carpet over to the window to see where Mitch is. She just walked over that spot a minute ago and didn't make a sound, but now they dub in the sound of her walking on a hardwood floor. (00:22:35)
Audio problem: After the gull hits her in the head, Melanie says to Mitch, "Yes, I think so," but her mouth does not go along with what she's saying. (00:25:35)
Audio problem: As Lydia talks on the phone about the chicken feed she says, "Well I'll try to go over and see him, maybe he ...". The next time she says something, her lips are moving differently. (00:35:15)
Audio problem: When we first see Melanie play the piano, the melody line that we hear is not what she's playing with her right hand. (00:35:40)
Audio problem: As Melanie sits and smokes at the piano, Cathy says, "Of course they're mostly hoods" but her lips say something different. (00:35:55)
Audio problem: The overdub of Melanie playing piano in the living room is the same volume when the scene changes to a different room as Mitch and Lydia talk inside the kitchen. (00:37:55)
Audio problem: When Mitch says to Lydia in the kitchen, "Yeah, I suppose so. Her father's part owner of one of the big newspapers in San Francisco," his lips don't match up with what he's saying and his voice sounds entirely different from his previous sentence. (00:38:10)
Audio problem: Talking to Melanie Annie gets up and says, "Afraid I'd give Mitch". This sentence is an obvious over-dub as the natural room echo that we hear in her previous and subsequent sentences disappears. (00:43:55)
Audio problem: As Mitch points with his left hand towards the beautiful view from the top of his hill, he is saying something to Melanie, that has been edited out. (00:48:20)
Audio problem: As Melanie walks into the classroom we hear the children singing "Risalty Rosalty" and as they turn around to look at her we still hear the song but none of them are singing. (01:08:55)
Audio problem: As Annie says to the students, "All right children now quiet," her lips do not match what she says. This continues before she says, "Miss Daniels" and her lips move and nothing comes out. (01:12:20)
Audio problem: After Annie tells the students about the fire drill, their lips, especially those of the girl in the front, are not in sync with what they're saying. (01:12:30)
Audio problem: As the birds sit on the monkey bars outside the school we hear the sound of many birds flapping their wings for several seconds before any take off. (01:13:20)
Answer: It is never explained, probably because it would ruin the mystery of the film to spell it out.
If you are a Hitchcock fan, you pay attention to dialogue. They mention quite often that it started Melanie's arrival, of which she brought 2 caged birds. The rest is implied.