We use an iPod Nano hooked up to an
iLuv portable speaker. It's not very loud (only really useful when climbing or cruising pretty gently), and certainly not waterproof (but niether is the iPod), but it is lightweight, pretty cheap, and available at the local electronics store. I made a bracket for that behind the captain's seatpost with a
Minoura water bottle cage mount that I already had and some all-purpose (plumbers) strapping. An old computer mount, some more strapping and a couple of zip-ties were sufficient to put a mount on the handlebars that the iPod slides in and out of, but is a nice tight fit.