Most cities have noise ordinances so you won’t want the audio for your projection to be too loud. One solution to this is to broadcast an FM signal so that people can listen to the audio on their FM radios in their cars. I use the one below:
Whole House FM Transmitter
This broadcasts an FM signal strong enough to pick up on the radio several houses away. It allows you to select the channel on which you want to broadcast so you don’t get interference from local radio stations. It also comes with a splitter so that you can send the audio signal from your player to both the transmitter and a stereo system.