Don't know what you're referring to when you say "creepy chant", but I did recognize the sample at 2:26! It's also in Koudelka. "BC AMENM03" from "Spectrasonics: Symphony of Voices", I think. If this is not what you were referring to, chances are still it's on Symphony of Voices.
Speaking of Symphony of Voices.. I just realized that probably most (if not all) vocal samples from Chrono Cross probably comes from it. Here is a couple I could place..
Chrono Cross- "LAMENT 07" from "Spectrasonics: Symphony of Voices" was used in 1.14 "影切りの森"
- "LAMENT 05" from "Spectrasonics: Symphony of Voices" was used in 3.05 "炎の孤児院"