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.
"The whole project started with the intention to manufacture and distribute electronics more sustainably," Harms says. Looking at this design, he nailed the assignment. The Boombox is made from .5mm thick stainless steel, cut by acid-etching, a low-energy technique that cuts the speaker holes and reduces the need for extra production materials.
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.