I've always loved to have music along when I touring. My old set up was a pair of battery powered speakers attached/strapped to my front panniers.

Below is a picture of my most recent set-up. The speakers are Logitech MM32. I had a local machine shop make this bracket for me. (to me it's a work of art, well worth the $80 the guy charged me to make it) The speakers slide into the hole in the middle of the speaker unit. (the hole is made to hold an iPod). I use a simple MP3 player that I have velcroed to the speakers. I like that I can easily slide the entire unit on and off the bike. The bracket attaches to the water bottle bosses.