In "To Kill a Mockingbird", what might be the cause of the laughter from inside the house?

As the previous answer states, the laughter from inside the house is probably Boo Radley having a good chuckle as he watches the antics of the children outside his door. Scout never lets on to Jem and Dill that she heard it at all; she keeps it to herself. This is probably because she is too afraid to even mention it. At this early stage in the novel, anything to do with the Radley house scares the children, certainly Scout, who is the youngest. The laughter seems scary to her.

It is not until the end of the novel, when Boo Radley is finally revealed for the eminently decent person that he is in saving the children from the depraved Bob Ewell, that Scout's perspective about him undergoes a sea-change. We get hints before this that the older and wiser Jem is already changing his ideas about Boo: for instance when he remarks thoughtfully that Boo's seclusion inside the house, hitherto such an intriguing mystery to the children, might be simply 'because he wants to stay inside'.

But for Scout, the revelation about Boo Radley is withheld until the end of the story. Then, it happens in a flash, and she comes to understand him and to literally see things from his perspective as she stands on the Radley porch looking down the street. She realizes now that Boo, or to give him his proper name, Arthur, has always been watching them and looking out for them, and that they also unwittingly helped him by affording him innocent amusement as he watched all their games. The laughter that Scout hears from the house earlier is an indication of the happiness that he derives from the innocent children playing in his neighbourhood. Although she doesn't know it then, it is a positive sign.