I’m looking for a book that I read in elementary school and was (I think) published in the 1980’s. It was a children’s chapter book. It was about a little girl who’s best friend was a boy. She had a crazy or strange family and they lived in a house that had secret hidden rooms or compartments. One of the rooms was soundproof and the girl’s uncle would play drums in the room. In one part of the story the boy and girl get locked(?) or trapped(?) in the soundproof room. Her family may have been spies or something like that. The word pepper keeps popping into my head and I’m not sure if that was someone’s name or in the title. I know it is not the The Five Little Peppers. Help!
I’m not sure about the word pepper, but many of the elements that you describe are in the beginning of the book The Magician’s Nephew by C.S. Lewis, one of the books in the Chronicles of Narnia series.
This may be a bit of a long-shot: The Mystery of the Secret Message by Elizabeth Honness. In it, a girl whose father has gone missing lives with her aunt and uncle in a duplex that has an elevator. She gets trapped in the elevator, which is in the wall between the two parts of the house and overhears her neighbors plotting against her. She thinks they are enemy spies. She makes friends with a neighbor boy and another girl who help her to solve the mystery. I don’t remember any real secret rooms or sound proof ones (or drums), though. Just the elevator and a treehouse.