Audio problem: In the first hijacking scene, where the Honda Civic darts under the trailer, you hear the car shift up 3 times. Each time, it pulls high revs, yet the Civic doesn't move forward (at one point it almost goes under the rear wheels of the trailer) or crash into the back of the main tractor unit. Surely, with the performance of these vehicles against a laden truck, shifting up three times would have caused it to hit the tractor unit. (00:02:39)
Audio problem: When Brian asks Harry what he said to Dom, As Brian says "Dom" his mouth doesn't move. You can see his mouth for the "Do". (00:09:30)
Audio problem: When Vince turns up at the BBQ he sees Brian and walks off - Dom shouts "Vince, give us a hand" and he replies "looks like you got all the help you need, brother" - his voice doesn't match his mouth.
Answer: There are two ideas of why the police did not come around. Firstly they might have found out after the car chase that Brian was an undercover cop so it would be more effective to allow him to try and discover information instead of going around and arresting Dom. The other idea is that the police might not have enough to arrest him. Dom was not the one driving when they found him and other than racing, it might be tough to arrest him.
Lummie ★
Cops are usually completely unaware of an undercover cop's identity. The more, realistic reason is that the cop didn't ID Dom and since Brian's car was later destroyed, there's nothing for them to investigate. Afterwards, they might ID Dom, but there's an investigation on him, so they'd leave him alone.