Noise enforcement is a normal thing in areas that want to enforce it. It happens exactly the same way speed enforcement happens, the officer has a device that measures it and can present that information in court if the other party disputes it.

The problem for both noise and speed enforcement is entirely a problem of will. Some jurisdictions simply don't care, or just do targeted enforcement to drive revenue but not actually change driving habits. Do any driving around the country and you'll see areas where people regularly drive 20mph over the speed limit and areas where people barely go 5 over.