How to power speakers with batteries without an inverter?

I have a powered speaker system that runs on 120 AC. I have the battery and the inverter but I know the speaker functions on DC and it has an built-in AC-DC inverter. So to use my battery, converting it to AC with an inverter and then the speaker converting it back to DC and all this process will result in a lot of loss of energy and cost me in terms of runtime. 
I would like to know if there is a way of by-passing the built in inverter and connecting the battery directly to the speaker without causing harm?
What I thought was using a multimeter, measuring the voltage and the amper output of the built-in adapter and buying a battery with the same numbers and connecting it directly there. What are the risks in there? because I don´t want to burn my speaker.

You'll need to make measurements. Quite often these things use split supplies, like +12, gnd and -12 - using batteries to replace them is very tricky.