Here's what happens before Ennis unhooks the drive-in speaker in the 2003 screenplay:
125 EXT: RIVERT0N, WYOMING: DRIVE-IN: NIGHT:
ENNIS and CASSIE sit in ENNIS'S truck, trying to watch THE EMPIRE STRIKES BACK.
CASSIE is rapt, eats popcorn, swigs cheap white wine from a bottle between her knees, despite a violent wind storm that has come up, blowing trash and dust into the windshield. The sound from old window-mounted speaker is barely audible over the windstorm. The wind is blowing dust so high into the air that the screen is partially obscured.
ENNIS
We ought get our money back.
CASSIE
(hits him on the shoulder)
Hush up, Ennis, this is important. He's learnin'
how to be a Jedi knight.
ENNIS tries to step the blowing dust from entering his truck cab, rocks an old flannel shirt in the crack created by the drive-in movie speaker.
ENNIS
My truck's fillin' with dust.
CASSIE
(rapt)
Shhhhi , Turn up the volume.
ENNIS
(fiddling with the speaker)
Up as high as it'll go.
A strong gust of wind carries a big panel of the screen away, leaving a rectangular hole in Luke Skywalker's face. Suddenly, ENNIS rolls down his window just enough to unhook the speaker and throw it out into the dirt. He drives off.