The Forty Part Motet is a sound installation by Janet Cardiff, a 40 track recording of Spem In Alium by Thomas Tallis presented at The Cloisters in Upper Manhattan by The Metropolitan Museum of Art. I heard about it from David Byrne:
In Cardiff’s piece, the singers—all 40 of them—are recorded, each on their own mic and track. In the installation, each individual voice is played back through their own speaker. The speakers are roughly arranged in a circle at head height. One assumes this is how they were positioned during the recording. The nature of the Tallis composition is that various groups of voices come and go, sing or remain silent for a bit: if you are standing by a speaker you might hear nothing for a while then suddenly a person’s voice booms out, with absolute clarity, as if they’re right next to you. So, depending on where you’re standing in the room, you hear a completely different balance of voices. Unless you’re in the center of the installation, you will hear some voices way louder than others.
There’s a moving review in the New York Times.