This looks good--
The article at Hack A Day links to www.instructables.com/id/Portable-Retro-Hi-Fi-Speaker/
Pertinent details: this speaker encloses a 12v 1.2 mAh sealed lead-acid battery. Also on the board is an amplifier built around a TDA2003A chip. Charger is an 18v laptop power supply. As usual with Hack A Day and Instructables--you should read the comments for possible improvements. I have not built this but I believe these instructions are a good place to start for an amplifier+speaker that would be loud and have some serious battery life. The second comment at Instructables suggests adding an integrated MP3 player--this might be a way to provide accompaniment.