This is really very cool. I stumbled across the YouTube page of a composer named Eric Whitacre
who put together not just one, but TWO, virtual choirs.
In essence, the "choirs" are made up of people from all over the world singing in polyphonic harmony. The "trick" is they recorded video/audio of themselves singing their specific parts and then the whole thing was cut together.
The stunning "Lux Aurumque" is embedded below. You can find the nice (but not as impressive) "Sleep" here