You are answering different question. What you are saying is called awareness campaign or something. Enforcement of seatbelts is done by police with fines/tickets and is possible cause it's visible from outside.

Other things like loudness levels inside cars cannot be monitored without going in full totalitarian mode.

Why would enforcement be necessary, given assumptions 1 and 2 (not stupid, not murderers), and awareness? Around these parts, seatbelt enforcement isn't necessary because everyone voluntarily wears their seatbelt – except for children, occasionally, but the adults are generally capable of enforcing that. (Even teenagers / young adults being irresponsible in cars generally wear seatbelts while doing so.)