A blurb at the YouTube page says:
This is part of the gregorian chant prosa 'Ave Maria'. Listen while watching the same in the original manuscript (E-Bc 911, which is a cantorale from Girona). This is from the cd 'Llibre Vermell de Montserrat' RIC 260.
This is an obscure text, as far as I can tell; there's not much in English on the web about it. Much of what Psallentes does seems to be taken from rather obscure old manuscripts; I'm really looking forward to learning more about the pieces they are singing. Here's their terrific YouTube channel.
(Amusingly, there's a Psallentes Ladies, too, on their own section of the the website. Here's what it says about them there:
Making a striking début at the Alden Biesen ‘Day of Early Music’ in 2007, a female version of Psallentes has arisen from the original. Eight young female singers devoted themselves to the interpretation of the ethereal music of Hildegard von Bingen. This may lead to a CD of the female ensemble, to appear in 2009. Meanwhile the project ‘Gregorian chant for young female voices’ made a second appearance with ‘De Begijntjesprocessie’ (Procession of the Beguines) a programme dedicated to the Gregorian chant of the Beguines, originating from various Flemish beguinages in sources from the late Middle Ages.)
Very interesting to me, the thing about the Beguines; I have a strong interest in one of those women, the 13th-Century Marguerite Porete (burned at the stake for heresy, of course). The Ladies are actually how I found this ensemble to begin with, after coming across their gorgeous version of "O Sacrum Convivium."