The Bike Boom Box started out in 2008 as a way to enjoy music on the Tuesday Night Bike Ride, a loosely-organized weekly ride in Tucson. I first tried using a regular boom box, but it didn't fit anywhere on my bike and it didn't have nearly enough power to be heard over any distance in the open air.
I remembered that I had that Burning Man amplifier board, and I built a wooden box to hold two 6x9" car speakers and the amplifier box in the top. The big wooden box was mounted on a rear rack and made my bike very rear-heavy, but it sounded pretty good and was loud enough to be audible to up to a hundred people on the ride.
I decided that the basic idea worked, but the box was too unwieldy. Some thinking resulted in the idea that 6" round speakers would fit in a piece of 6" ABS drain pipe. That would be a lot more manageable, and look better to boot. I went to the industrial plumbing supply house and bought ten feet of 6" pipe.
I made a 16" long tube and installed a 6" car speaker in each end of it. The TI evaluation board was installed in the tube with a charger port. I bought a 14V, 2AH lithium-ion battery battery pack and charger from batteryspace.com to make it lighter weight. I mounted this tube to the back of my bike using a couple aluminum straps and rods, which worked for a while but was not very reliable in the long run.