What do the boys hear that stops their argument?

Chapter 26

On the way back, the ambulance pulls up next to the police car and one of the interns says that he thought the old man was dead. The police think he is joking, and in the back of the police care Will and Jim cannot say anything more. They get dropped off at two random houses close to the police station but do not go in. Will sees that Jim still holds his tickets for the free rides, even though he threw his away. Will wants to tell someone in authority but Jim convinces him that they have no proof. Jim thinks that it would be all right if they could apologize to Mr. Cooger, and Will explodes. He cannot believe that Jim is not afraid, why he does not seem to understand the danger. Will argues with his friend, telling Jim he does not really want to be older, but Jim does not agree. Will points out that Jim will leave him as soon as he is older, and that he is glad the machine does not work anymore, and Jim is angry with Will for destroying the machine. Their argument ceases when they hear voices nearby and stop to listen. Miss Foley is talking to Will's father about the break-in at her house. After a few moments of speaking she asks where the boys are if they are innocent. Will jumps up and climbs through the window after he hears her question.

Their argument ceases when they hear voices nearby and stop to listen.