On a recent ride (the STP) there was one group of guys who had one of their riders towing a BOB trailer in which was built out of plywood a boombox that included a car stereo and presumably some kind of internal power source. Attached to the outside of the box were car audio speakers. He controlled the system with a wired remote control complete with detachable connectors that was attached to his handlerbars. There must have been some kind of hi-capacity MP3 player in the system too since he never ran out of tunes and was actually taking requests (he filled a good majority of them). Needless to say, he picked up quite a rolling crowd. I must have followed him for a good 30 miles at first and managed to regain contact with him several other times during the ride.