Whatever it is, i'd also have preferred it without this effect for the whole song, and not just in some parts. I still like it a lot though.
And i love the 3rd song on Azusa's CD, (kuon no kawa, 久遠の河). Her voice is great for ballads. I'd love to hear Azusa sing some of Chihaya's songs like Yakusoku, Saihyou or Nemurihime