pirate radio stations often make use of channels that otherwise have other content being broadcast on them. most such stations cover very small distances, but within those ranges, they become the dominant stations.

as such, you could be listening to NPR while approaching that house, then suddenly be serenaded with an adventurous-sounding theme as you passed by. if a given pirate happened to be feeling particularly naughty, they could research the most-listened-to stations in their area, set up their system to locally dominate the requisite channel, then broadcast either police sirens, loud horns, or selectively chosen clips of hardcore pornography.