Samuel Barber is one of the best-known American composers, and some of his works have entered the standard repertoire of classical orchestras here and overseas. Chief among them is undoubtedly his Adagio for Strings, originally written as the second movement of his String Quartet in B minor, Op. 11, composed in 1935-36 and revised until its final version in 1943. It was played over almost every American radio station when the news of President Franklin D. Roosevelt's death was broadcast in 1945, and is still used as mourning music to this day.
The Adagio has been adapted in many ways since its origin. It began as music for a string quartet, performed here by the Dover Quartet.
In 1967, Barber adapted his Adagio into a choral work, the Agnus Dei (Lamb of God), part of the traditional Latin Catholic Mass, with mixed chorus and optional accompaniment by piano or organ. This performance is by the British octet Voces8, sung a capella without accompaniment.
There have been other adaptations over the years by other composers or arrangers, none of which, in my opinion, measure up to those of its original composer. It's a lovely piece, well deserving of the acclaim awarded to it and Samuel Barber over the years.