The film is the one the OP posted.I didn't know there was a film. It was a story I read about Neil. On a mailing list...they had t-shirts made up. Maybe we're talking about two different things?
The story is about Neil taking someone out in a boat on his pond with speakers set up to listen to a mix, one of the stacks was in the barn. It wasn't as loud as the other so he yelled to his engineer 'Needs more barn!'
That's the story I was thinking of, anyway.
Went looking, it was Graham Nash.
"In September of 1971, Neil Young took Graham Nash out on his lake in a rowboat and blasted his new album, Harvest. He’d wired his Northern California ranch house as the left speaker and a barn as the right speaker; as the record played, his producer, Elliot Mazer, ran down to the shore to ask how it sounded. “More barn!” Young famously yelled."
- D