Sing We Christmas by Chanticleer is an a cappella album.
from the little booklet that comes with the CD:
This recording offers a sampling of the listening experience enjoyed by appreciative crowds who flock to Chanticleer's enormously popular Christmas concerts each year.
from Chanticleer's web site:
Recorded in a beautiful church in the old walled city of Kempen during a November 1994 tour of Germany, Sing We Christmas represents many western European traditions and a great variety of time periods, languages and styles. A wonderful collection offering traditional and contemporary settings of holiday music, including anthems, canons, carols, dances, hymns and motets.
O Magnum Mysterium is on the album:
You can listen to some of the music at the Amazon.com page.