It's Barber's "Agnus Dei" - on the CD "Evocation of the Spirit",
Robert Shaw and Robert Shaw Festival Singers.  "Adagio for Strings" was
origionally part of an early string quartet.  Toscanini convinced Barber
to take the slow movemont of that quartet and make it a piece for String
Orchestra.  Later Barber took the work and set the traditional "Agnus Dei"
to it.

=Dave Harman=