Here's a blurb I posted this morning, at Film Score Monthly, about the Hollywood Bowl concert of Jurassic Park. Wrote this before discovering this thread; now edited my copy/paste, to focus on my main query:
I had the pleasure of experiencing "Jurassic Park" with, I estimate, maybe 15,000 people at The Hollywood Bowl where the phenomenal crusader of film music David Newman conducted the LA Philharmonic playing John Williams' score live to picture Friday evening. It was terrific fun.
I know, more recently, there have been many older live-to-picture shows – "The Wizard of Oz" (1939), "Casablanca" (1942), "Vertigo" (1958), "Psycho" (1960), "West Side Story" (1961), "Silent Running" (1973), "Jaws" (1975), "Close Encounters" (1977) and soon we will getting "Superman: The Movie" (1978).... So, how do they do that?
Wikipedia says projection material is prepared with 'music suppressed,' but that sounds like hand-waving to me. In my experience, movies are not made that way. No one had any reason to make a music-free mix of their film, back in the day. What would be the point? And surely all the audio stems would have been long lost, resulting in most cases in mono magnetic or optical audio tracks, which would surely be impossible to pull apart.
Bravo to all involved in producing these events, all over the world: https://filmconcertslive.com
And what a phenomenal job David Newman does. He is a marvel.
TL;DR edit:
How do older films create music-free audio as playback for live orchestral performance?