We love electricity-free speakers; once made, they stop consuming resources and keep pumping out sound. If you're going to go electric, though, there are good choices, including this super sleek option: the Flat Boombox. The work of Hannes Harms, a student at the Royal College of Art in London, the "boombox" is not only as thin as a credit card, it's made with some impressive sustainable techniques.
More impressive is that this is actually a flat pack design. The four components could be mailed in an envelope (reducing shipping costs and resource use); they just have to be folded a bit and assembled to make up the speaker.
And while Harms' Flat Boombox does require electricity, it's small enough that I'm sure you could hook up your amplifier to run on solar power.