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We can infer from the following quote that the laughter was in fact Boo's.
"The first reason happened the day I rolled into the Radley front yard. Through all the head-shaking, quelling of nausea and Jem-yelling, I had heard another sound, so low I could not have heard it from the sidewalk. Someone inside the house was laughing." (Chapter 3/ Page 22)
To Kill a Mockingbird