Here's the note from the YouTube page:
Lapidem, quem reprobaverunt aedificantes, hic factus est in caput anguli: a Domino factum est, et est mirabile in oculis nostris.
Psalm 118(117): 22.23
The stone which the builders refused is become the head stone of the corner. This is the Lord's doing; it is marvellous in our eyes.
"Lapidem" is the second verse of the offertory "Benedictus qui venit" sung by Geert Maessen in the Amsterdam Obrechtchurch on June 17th, 2009.
Saint Gall neumes: tenth century, CH-E 121, p. 224 and CH-SGs 339, p. 113. Fluxus score: 2009, Geert Maessen, Amsterdam
Interesting to see, first, the old-style neume notation in the manuscript, and then the modern rendition of it. See "lelalilu"'s YouTube channel page for more.