'Brother Martin, Servant of Jesus' is indeed a 35mm nitrate print of the trailer for the feature film which doesn't exist. It might sound sacrilegious but I don't think there is anyone alive that truly mourns its loss.
The other two are the well catalogued Vitaphone discs, not the films, that were donated by Warner IIRC quite some years ago. These are part of a very interesting collection that includes the Ernst Lubitsch 1928 film The Patriot that was shot as a silent but then had a new soundtrack created, and from the same year the D.W. Griffith comedy 'The Battle of the Sexes'. These both have a very interesting history, but the rest of the collection is also very enlightening as you may discover if you look into it further. Re: Lilies of the Field; you neglected to mention that the trailer for that is also there, albeit in Vitaphone sound only. Oh, and the BFI hold some Technicolor nitrate of 'No, No, Nanette. Just 160ft so probably just one short scene.
All you need to do now is find the actual films and then you can marry up the sound!
Brilliant findings! How on earth do you find these treasures? Im very impressed. Although if the BFI gets their hands on these they may as well stay buried. Wonder how these films ended up at UCLA? Again brilliant work Ray.