In 1957, Maria Callas sang a concert with the Dallas Symphony under the baton of Nicola Rescigno. Someone snuck a tape recorder into one of the rehearsals for this concert, and the resulting "Callas/Rehearsal in Dallas 1957" made the rounds of reel-to-reel tape-traders back in the 1960s.
David Abramowitz, my very first opera-friend, gave me a copy of the rehearsal tape and I enjoyed it, despite being somewhat frustrated with the stops-and-starts as Callas and Rescigno worked out the interpretive details. I was especially impressed by the different takes on passages from the entrance scena - sometimes referred to as the Letter Scene - of Lady Macbeth from Verdi's MACBETH.
It occurred to me to patch these phrases together and create a complete run-thru of the recitative and aria. Years later, when I was getting rid of my reel-to-reel collection, it was one of the few things I saved. The voice of Maestro Rescigno can sometimes be heard, and there's some static at first, and a bit of tape drag. But once she's into the aria proper, it gets better.
Maria Callas - MACBETH aria - rehearsal composite - Dallas 1957