Well, during my search, I discovered that "The Sound of Music" was NOT the first movie about them. There was an earlier German movie, "The Trapp Family In America".
However, I wanted to find something by the real Trapp Family, not by actors portraying them, no matter how beautifully.
Well, I found a little gem!
And as if this weren't enough, I also came across, as a wonderful bonus, two clips of
Maria Augusta Trapp with Julia Andrews! And yes, they sing together!