Just to make sure you know what you should be doing, you need to think as those 6v batteries as half a battery for your purposes. You will take those (4) 6volt batteries and make (2) 12 volt batteries by connecting them in (2) series pairs. To make one 12v battery, connect the + to the - of the other. if you use a multimeter(if you dont have one, stop right there and go buy one now) and measure the volts between the unused + and - , you will read 12 volts now. Repeat the above steps with the other 2 batteries. Now you have (2) 12 volt batteries (effectively). Now to connect those together 12 volt sources together, wire them in parallel, connect the unused - to the unused - and the unused + to the unused +. now you have an even larger 12 volt battery. PLEASE check this with a multimeter before you hook them to ANYTHING including the ground on your vehicle. Your equipment and maybe your eyes depend on it. Here is a good link to some reading on setting up battery systems. I highly recommend you read and understand what you are doing before messing with this much current.
I am also curious about your musical equipment. Everything I have ever worked with was 120 volt. Are you sure about the 220? If so, you will have to spend a lot more on an inverter to run 220. Usually you have to have 2 rather expensive inverters stacked or I have heard about converters that raise the voltage. Do you have the equipment already? If so what kinda plug do they have(number of prongs, shape . . ). Good luck.