PLEASE: Post your ideas to add on to this in the comments section, for everybody else, and me, to do. There's never too much music or speakers to blast while biking!
Step 1: THE INFAMOUS PARTS LIST
A cheap mp3 player, it will be alot easier if it takes a disposable battery...
A ton of duct tape
An old speaker... I used a speaker from a broken amp
Some batteries, the bigger (D) the better
A speaker cable
NOTE: You're mp3 player might break, thats why the cheaper the better...
Step 2: WHAT TO DO
Now, take your speaker and connect the stripped wire to it, and plug the other end of the cable into a computer headphone jack. Switch the wires around to the opposite sides and see which sounds better. Once you have decided the better sounding way, solder or just tape them together.