Actually, listening to it, it
really sounds like they're just singing syllables in much the same manner as Adiemus.
Take a listen and see what you think.
At any rate, if it's not that it's obviously a language without consonant clusters and I very distinctly heard an interdental fricative in it, which really, really narrows it down. I'll look on WALS to see what I can find that possesses those two qualities.