Audio problem: When Kevin plays the tape of his Uncle Frank singing in the shower, it starts with "We know a guy that can really do the cool jerk" twice. Yet, when he first tapes his Uncle, he doesn't start the recording until after that has been sung. The recording also sounds very clear. When he tapes him the door is closed, but the tape sounds the same throughout. Also, when his Uncle says he's gonna "slap him silly" Kevin shuts the door behind him and you can hear it shut. On the tape there is no sound of the door shutting.
Audio problem: When Kevin is singing on stage and Buzz is playing air drums by Kevin's ears, Kevin's singing is out of sync with the audio.
Chosen answer: Remember that Kevin didn't give his name, nor did the police ever see him. When he called, it was from a different address and he gave his name as "Murphy." There is no reason to think that Harry and Marv would tell the police about Kevin, since they would have to admit being outsmarted by a child and also admit to kidnapping, assault, battery, and a wide array of charges for what they tried to do to him. Even if he did testify, there is no reason to think his parents wouldn't know.
And by the way Kevin knew nobody would believe him.
Not entirely correct - He rang the Police from the phone in his parents bedroom just before the spider scene.