I would just muster up the courage and ask them to cut it down nicely. Music that loud can damage their hearing so they probably don't know how loud it is.
Seriously though, I would be more upset if someone got the police involved than if they just came over and asked. If you ask and they still keep at it, THEN call the police. Of course, then they would know it was you and that wouldn't be good either. Ugh...no good way out of that I'm afraid.
And speaking for myself as someone who has that 'stupid loud radio stuff in her car' let me just say that it is extremely RUDE of your neighbors! They know good and well that is disturbing. I would never DREAM of playing my stereo that loud near houses. When I turn off the main road, the radio goes off.
