I ride the San Diego Critical Mass and there's always this middle age gentleman who rides a recumbent with a sound system that might interest you. He uses Yamaha Speakers that were meant for computer systems. Decent sound. The beautiful part is that you can power them with 12Volt batteries which will last long and are readily available. I haven't noticed how he keeps them waterproof. but Then again, I live in Southern California where it almost never rains.