..........here is an idea off the top of my head,i just woke up so this maybe rough.... first off i would try building the electrical charging system located here http://motorbicycling.com/f4/6-volt-...ll-3871-3.html
thats gonna give you a nice 6 volts, but not to many amps to mess around with. next i would buy a cheapo boombox that runs off batteries (generally these run off 6 volts) find one that can use a flash drive/usb for storing you music on without worrying about skipping from shock like this Portable CD/MP3 Boombox with Memory Reader - Boomboxes, Portable Audio & Video, Audio & Video, Electronics & Computers - nuLime.com
... then i would pull apart the whole boombox keeping the circuit board and certain parts,cut out the cd player cause im not gonna use it and then build a custom encloser for it,mount it to the handle bars and wire it up to the battery on the bike. then take the speakers from the boombox and mount them to the bike.
the boombox wont take as many amps to run as a car stereo so in theory it should work and as long as you know what wires your cutting on the boombox it should work without the cd player part.
now this gives me an idea for my next build like a harley full dresser or honda goldwing,lol