It’s a pretty solved problem. In NSW, Australia we have automated systems for heavy vehicle noise fines. (They could of course be used for regular vehicles as well, we just don’t)
For all vehicles the police can issue a defect notice to anything that seems too loud. The owner then has to present to an inspection station and prove that they’re compliant. If they don’t show up or don’t test as compliant their registration is voided and they’re fined. Of course, they can remove the modification before visiting the induction station then reinstall it afterwards, but that gets pretty time consuming pretty fast and people largely stay within the bounds of reason as a result. The system isn’t perfect but it works fine.
For all vehicles the police can issue a defect notice to anything that seems too loud. The owner then has to present to an inspection station and prove that they’re compliant. If they don’t show up or don’t test as compliant their registration is voided and they’re fined. Of course, they can remove the modification before visiting the induction station then reinstall it afterwards, but that gets pretty time consuming pretty fast and people largely stay within the bounds of reason as a result. The system isn’t perfect but it works fine.