The boy said, "You are thinking of something you thought I wouldn't guess". The boy was right by technicality.
There are two ways she figured out: 1. He would have had to hear her to know she was talking as he was focusing on driving. 2. He would not have heard her directions if he was deaf and wouldn't have nodded in response as a way to confirm he got the directions.
Jake is a schizophrenic who hallucinated beth, having her guess right every time.
Because the man would not be able to end/wind the tape, meaning the tape would still have been recording,