There are no laws anywhere in the world that make it illegal to play loud music in a car.
However if the music is deemed to be causing disruptive or antisocial, or if music is being played late into the night in a residential area then the authorities can step in.
There are no laws anywhere in the U.S that put restrictions on the decibel levels of music you play in your car.
Instead, states may have laws that say music from a car stereo must not be audible more than a certain amount of feet away from the vehicle.
Check the state regulations in your area by searching "car stereo regulations state name".
It can be difficult to ascertain whether or not your music can be heard at these distances without running a few tests on your own car at different audio levels.
Even the most thoroughly soundproofed car will let some noise out, every car has a different level of sound resistance. Testing is important to ensure you don't fall foul of state regulations if you plan on playing loud music in your car.
Whether or not these rules are applied depends mostly on intent. If you weren't aware of quite how loud your music was then you will just be told to turn it down a bit however if you are purposefully being disruptive then you may receive a traffic violation.
If you are being frequently distrubed by loud music from a car then contact your local authority with details including the location, times of the incident and a recording or video if possible.
In the U.K your local council may intervene if the car is stationary in a residential area.
Police officers may also intervene if they witness excessive music noise from a car if they deem it to be distracting or a nuisance to other people in the area.
Generally, local authorities are unlikely to do anything for a one-off incident however if it is a regular occurrence then you should inform your local council.
Noise complaints are handled by local councils, if you have a serious noise problem and want action to be taken then you need to report the noise to your local council.
In order to help your claim there are a few things you could do:
Noise problems can usually be reported through your local authorities website so just do a google search for "your local authority name make a noise complaint".
If you enjoy playing loud music in your car here are a few things you should be aware of to avoid getting into trouble or becoming a nuisance to anyone else: