Mass X is called Alme Pater. My first exposure to its haunting melodies came from a polyphonic setting written by Wilko Brouwers, presented at the Colloquium some years ago. I fell in love with that piece, but it was another year or two before I encountered the foundational chant on which the polyphonic piece was based. Then it was love at first listening.
For those who wonder why it took so long to hear the original Mass X and find a copy, we have to remember that Gregorian music was not really available then as it is now. There was no Liber online, no Kyriale, no Parish Book of Chant, no thousands of youtubes of chant settings, or anything else. Most of this music was obscure and hard to come by, or expensive and difficult to acquire. Today all that has changed, with resources dancing all over the web, begging for attention.
In any case, this morning at Mass, our schola resurrected the Sanctus of Mass X to great effect. It strikes me that this setting is extremely parish friendly, mostly syllabic but with a profoundly affecting theme.
Here are some scores of the full setting. Be careful of the first MP3 on the Sanctus because the singer actually sings a wrong note in the second phrase. The second MP3 is correct but not a great recording. It seems that there is still work to do in making this material even more accessible.