Here is some rare footage of musicals from the past. The best way for them to be preserved through the ages is to share them amongst people wo appreciate musical theatre, so here they are. Most are silent home movies shot by people in the cast or by audience members. If you go on my youtube channell, there are many more, but here are some of the most interesting ones:
In response to Mr. Roxy, I believe that they didn't actually sell them, but just kept them to watch at home (possibly shared them with others if they were collectors). However, most of these home movies were shot by a journalist, Ray Knight, who would come once or twice a year to Broadway and filmed every major (and not so major) musical. He filmed over 175 Broadway musicals, starting in 1931. He caught on tape shows such as Anything Goes (with Merman), Damn Yankees (Gwen Verdon) and Camelot. After he passed away in 1973, his collection was bought by collector Miles Kreuger. Sadly, he won't share this precious footage, and it is slowly decaying in a cupboard in his home.
It's because there was a different 'mentality set', a different way of thinking. There was also the problem that home movie cameras were rather big and noisy, so not exactly easy to hide in a theatre. Ray Knight usually filmed during tap dances or chorus numbers for fear of being discovered.
However, a lot of the films that are available were filmed by cast or staff members of the musical: Don Pippin (famed Broadway conductor) filmed Mame, Dear World, Mack and Mabel, The Grand Tour and La Cage Aux Folles, while Candy Brown (chorus girl) filmed Chicago and Pippin.
I assume you meant audio/video together, but I believe the oldest audio bootleg is of THE ZIEGFELD FOLLIES OF 1934, on tour in 1935 with the original stars. It was actually released (though in truncated form) on CD some years ago by AEI. Impossible to find now though.
R&H filmed the original 1951 West End cast of "South Pacific" starring Mary Martin and Wilbur Evans. Not strictly speaking a bootleg, but hey. https://www.youtube.com/watch?v=GGdp4nVItqM