I know the idea of boom-boxes has been tossed around before, and I think that would be really cool.
I would -really- like to see it come through in cars though. Cars are a huge investment and being able to bump some tunes would be great. Maybe you could buy a stereo system and have a mechanic install it? This could also be something for people to steal and re-sell. The system could be turned up or down so that people could hear the music in the cab, in the backseat, and outside sort of like the car is emoting loudly.
Maybe deckers could get some blanked gear and record for musicians? Something more for deckers to do would be fun, and having live recordings of musicians would be really fun.
"Oh shit, I love this track, they recorded it live at O, remember? After this verse is when those batas came in with the uzis and started shooting up the place, and this bitch just kept on rocking!"