It appears that producers have auto-tuned certain voices, I mean just listen to the female from G&S during Don't Stop Believing, you can CLEARLY hear that her vocals have been subject of auto-tune.
http://www.youtube.com/watch?v=pvSEpZEAqfY
Also, during Shirlena Johnson's audition it is clear that producers have auto-tuned her vocals between 2:19 and 2:45.
http://www.youtube.com/watch?v=ff7SkPF8IuQ