A while back, oldbob posted an impressive choir simulation using a vocoder and sample library. Damned if I can find what forum he posted it in. But for the hell of it, here's a different way to go about it. I'm playing around with a TC Helicon Quintet (now discontinued, but they have other models) which is basically for adding harmonies or doubling voices to real singer's vocals. But this unit can be used by a non-singer by talking into the mic while simultaneously playing the desired notes on a midi keyboard. So I'm trying to see how well it can mimic a choir of male/females voices.
The mp3's below are just some initial tests. I'm gradually trying new things to hide the synthetic or mechanical nature of voices, but if mixed in with other instruments, some of the flaws can obscured.
This file shows some techniques for loosening up the sound and generating a real-time choir:
Simulating a boys choir:
Multitracking monophonic voices one at a time on 12 tracks for some nice polyphonic vocal work: