you must have one hell of a sound system! Usually it just means your battery stinks and should be replaced.
But If you insist on a second battery electrically, to hook up a second battery you would just put it in a parallel circuit. That is exactly what you do when get a boost from someone when your battery is dead at the shopping mall. Neg to Neg and Pos to Pos. If your put your battery in series your Car voltage would be 24Volts (aka not good).
The only problem would be finding space to put the second battery.
hope this helps.