Cables most likely aren't going to solve your problem - the issue is simply generating the power you need to run that load. If you really want to verify, setup the car in the condition where it's happening (like engine idling in gear with AC on and stereo going), then check the voltage at the alternator, then at the two batteries. If the voltage is significantly higher at the alternator than at the batteries, I'd check and clean all the connections first, and see what that does. Then, if you are still seeing a significantly lower voltage at the batteries, I'd replace the cables.
Otherwise, I still maintain that the cause is with the power generation (alternator speed), not the distribution (cables).