From the YouTube page, this is:
The first movement from Gabriel Fauré's Messe de Requiem. Performed at the Solemn Mass on All Souls' Day, 2nd November 2011. The choir of the Church of Saint Mary the Virgin, New York City, James Kennerley, Organist and Music Director. This version is an excellent arrangement by David Hill for organ, violin, 'cello and harp accompaniment. It is published by Novello.
All the videos are here, on James Kennerly's YouTube page; here they are, too, each linked below:
Here's more about the Fauré Requiem. The movements are in the following order (links below to Wikipedia articles about each item):
- I. Introït et Kyrie (D minor)
- II. Offertoire (B minor)
- III. Sanctus (E flat major)
- IV. Pie Jesu (B flat major)
- V. Agnus Dei et Lux Aeterna (F major)
- VI. Libera me (D minor)
- VII. In Paradisum (D major)
Here's more about Requiem masses on this blog. And here's the complete All Souls Days Office.