The best way is to tune into the radio station they broadcast the movie audio over.
As others mentioned, you can hang an old fashioned speaker just inside your window, or hear the movie from the speaker while sitting outside your car in camp chairs or on blankets. We usually do the radio station option and open our back gate and sit just behind our car.
There are also plenty of rows with speaker boxes you can pull up to!
Portable radio works good too
Through you car speakers, they give you a radio station. Don't have any interior lights on, put your car on ACC, one click to the right without any dashboard lights on, and you'll get radio and have jumper cables in case your battery dies
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