Ikatsui Senerio
From my understanding, there are at least singers for songs such as Life Returns and Dawn Awakens. You can look up "Vocals" under the credits to each song.
In Path of Radiance the vocals were Masatsugu Igarashi and Noriko Aoki. Vocals for Radiant Dawn were Toshinori Shimizu and Noriko Aoki.
I do believe, though, that songs such as Rafiel's Aria, Grace, and Repose of Souls seem the work of computers. Grace does the most; should one listen to the song for prolonged periods of time, one would notice that Leanne never pauses to take a breath. If she did, the echo covered it up nicely. I'm not so sure on Repose of Souls — that one seems more likely to be vocalized. Life Returns and Dawn Awakens are the big ones. Time to look them up.
As for Grace, Rafiel's Aria, and Repose of Souls... Yeah. As I said, I can tell that there is computer manipulation at work, what with the lack of breaths and other effects. I wonder if they were completely computerized or if they were actually sung, but altered later. I guess the lack of credit suggests the former, but I haven't heard computerized voices of that caliber.
Edit: Ah hah! I found Toshinori Shimizu, Yoko Fujii, and Aoi Tsujiyoko credited for the other songs in Radiant Dawn. I can't find any other vocal work from these people, but I know which names to listen for now. Thanks for the help.