No need to worry about batteries when you take along the Bird-Electron EZ-TAKEGTF2 electricity-free portable bamboo speaker ($199). Just plug the stereo miniplug into your MP3 player and you can rock the bamboo, off-grid style.
And bamboo, is, you know, sustainable. Get yourself a solar power unit for that iPod, and you'll be set to hunker down all the way through The End Times, where the definition of "sustainable" may mean lasting into the next day. What song will you listen to during the upcoming apocalypse?