I found out about this recording on Facebook. Eric Whitacre is a composer, lecturer and conductor. The virtual choir concept came to be when fan of his uploaded and shared a Youtube video of her singing one of his compositions. That snowballed into a worldwide virtual choir. People from around the world auditioned by submitting their Youtube performance of their part, recorded while following the audio track and video of Whitacre conducting. One person came forward to clean up the audio and put the 2000+ videos together. The result is astounding. Magical. This one is called "Sleep": Check out Whitacre's Youtube Channel. Also, here is his introduction of the process:
Now, what I want to know is when Virtual Choir 3.0 is being put together. (c) 2011 by Musespeak(TM), Calgary, AB, Canada. All rights reserved.