On a philosophical level, this question fascinates me. I think about it a lot, not only in terms of the specific context above (recording/filmmaking in the 50s/60s) but in terms of almost any form of expression, in whatever media.
How many stories never committed to paper?
How many songs never performed?
How many dances never danced?
How many films never filmed?
I can never escape what Joan Didion said: “We tell ourselves stories in order to live.”
For the readers of writers like Borges or Akutagawa, you might be able to begin to imagine what I’m getting at here.