If you look up the tune for “Where Charity and Love Prevail,” it says the tune is CHRISTIAN LOVE by Paul Benoit, 1961. There’s an earlier version from 1959, too.
But no, that’s not the whole story.
I am listening to Frofro, a medieval Christmas album by the Ioculatores and the Schola Cantorum Leipzig. And they are singing St. Ambrose’s famous song, “Veni Redemptor Gentium,” to the same tune, which was a 12th century tune found in a manuscript. (Einsiedeln, Stiftsbibl. 121). The chant tune was known all over Europe, and came down to the present.
Apparently this is a known fact, because it’s on the Wikipedia page for “Veni Redemptor Gentium.”
But wait, there’s more! Luther’s translation was sung to a “simplified” and hip 1500’s version of the same tune, known today as NUN KOMM, DER HEIDEN HEILAND. (Often used for the English translation of the Luther translation, “Savior of the Nations, Come.”)
It does not sound like the chant tune VENI REDEMPTOR GENTIUM. I mean, it sounds like somebody beat up the chant tune, if you have a lot of imagination. (I like the NUN KOMM tune, though it is very German.)
Meanwhile, “When Charity and Love Prevail” is Omer Westendorf’s translation of the Latin chant hymn “Ubi Caritas,” so it makes sense that they’d pick a chant tune. Just not the normal chant tune for that particular text.