I think Mitsuda used a vocal sample for Xenosaga's "Song of Nephilim" as well.
Yup. I've heard the same sample used in some pretty random places, though I can't cite specifically where from memory. The first three notes (two eights and quarter) are one sample and stay the same, while the second note seems to change depending on the source. Sometimes it's omitted altogether (check Xenosaga II's movie scene tracks by Yuki Kajiura). All it is is a sample of a boy soprano singing.
Finding the sources of the vocal samples employed by Mitsuda across his discography would be a study in itself. Considering his fondness of ethnic music from all sorts of cultures, we'd be facing quite a challenge.