If you don't want to use really heavy deep cycle lead acid batteries, (lithium-ion and lithium-polymer) batteries are expensive and also DANGEROUS unless they have the correct charger and you know what your doing.
Maybe just get this:
It's the Dayton Audio DTA-1 Class T Digital AC/DC Mini Amplifier 15 WPC
And runs on 8x AA batteries or can be AC powered as well. I would use 8x nimh rechargable batteries in there.
I like the Sanyo Enloop ones the most for high drain applications.
Speakers I'd suggest 2 of these for it: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=295-010
Dayton Audio PA130-8 5" Full Range PA Driver
Nice and efficient at 90db at 1w/1m, so would get nice and loud with not much power.
And they go from 90hz-15khz.
If you want extra sparkle a pair of piezo tweeters with a 3 watt(or higher) 10 ohm resistor on the positive terminal side of the tweeter which tones down some of the harshness and stops the amp from osscilating. Also as the frequency rises, the impedance continues to drop on a piezo so can damage the amp if it drops too low.
Goldwood GT-1005 Wide Dispersion Piezo Tweeter