Hi there, if you want to run sound from batteries your gonna need quite a few!
-a normal car battery has 40 amp hours (will run 40 amps for an hour or 1 amp for 40 hrs)
-a car battery is as you know 12volts
-the power of most soundsystems is rated in watts, which
is the number of volts mulitplied by number of amps
– so a car battery will give you 40amps x 12v = 480watts but only for 1 hr per battery – this is the maximum power you’d get out of the battery you could run it at 100 watts for nearly 5 times longer, but this still doesn’t take into account lights, computers, cd players etc that you might use..

if you want more help me an fruitisbad (posted above) are trying to knock something similar together so we could maybe swap ideas..
let us know if it works!!