Conversation so far:
how do you hear the movie at a drive in theater
The movie's sound is provided by broadcasting the soundtrack at a low output power on AM or FM radio to be picked up by a car radio.
How many drive-in movie theaters are left?
There are currently only about 330 drive-in theaters that remain in operation in the United States compared to a peak of about 4,000 in the late 1950's.

Question: Why did the theaters decline?
Self-contained rewrite: Why did drive-in movie theaters decline in number?