How can I play music inside the car with the engine off and not drain battery/

Simply put, it's not that possible to do this. There is so much power being drawn by the sound system and related electronics that there simply isn't enough reserve capacity in the battery to support doing this for long. If you hooked up a battery charger to maintain a charge, this could be possible, but I think that probably defeats the purpose.

Home stereo.... Boom box ..... Nothing's free!