I am wondering if they broadcast the movie audio through a PA and if the quality is decent. Alternatively, what is the best way to hear the movie in the bed of a truck.?

You watch the movie and listen on your FM radio station, so it as decent as youe car stereo

We back our pickup in. We don't have a sliding glass back window so we bring a portable radio. The audio works fine. There are no PA's, just your car stereo

If you didn't have a car stereo (or it shuts off automatically if the car is not running), you could listen on a portable FM radio, with or without headphones.

To listen to the drive-in you need a real FM radio that can tune to 100.7 as an over the air broadcast. The drive in in not broadcast on the internet or via satallite radio. The range you can receive the broadcast is less than a quarter mile