This thread reminded me that I have a small set of powered speakers that I have always wanted to attach to a bike, but could never think of a way. Well, after adding some baskets to my mountain bike I realized I solved the attachment issue:
Its an "iPal", got it for Christmas or a b-day one year. Hooks up by cable to my mp3 player. Not as big or elaborate, but will help make the ride to and from work a little more interesting.