This arrangement is very well-known in choral circles and is a festival / processional setting of the 14th-century traditional Latin hymn.  The work is available in two voicings: SATB [ RBM - 005 ] and TTBB [ RBM - 005a ] .  In its original form, the accompanying ensemble is 4 trumpets and 2 percussionists with organ; there also exists a full orchestration (WWs in 2s with mandatory Organ), created by the arranger.  (Both accompaniments are available for purchase).  There are also detailed instructions available for staging the piece as an actual processional in a concert hall or church setting, available on request from the distributor.  

The full orchestral version of this piece has been commercially recorded by the Dallas Symphony Orchestra and Chorus on their 2000 release "Dallas Christmas Gala," Delos # 3267.