How it all started: I ride my bike in a community ride every week, and the folks there wanted some way to enjoy music on the ride. I tried a regular boom box, but it's just not made for bike mounting. Being an engineer, I decided to make my own bike-mounted sound system. This is what I came up with.
Take a look at http://www.cathodecorner.com/bikeboombox/
showing the system in detail.
The amplifier circuit board is of my own design. It uses a Texas Instruments Class D stereo amplifier chip to provide 15 watts per channel of clean sound with very low battery drain. I designed a circuit board with everything needed for a nice bike system, including a charging port for your MP3 player.
The amplifier board details are http://www.cathodecorner.com/bikeboombox/cdamp/