Voice is the original instrument. Instrumental music, per se, in the Renaissance was largely derived at first from vocal lines: chant and harmonies.
Which O Magnum Mysterium? Lauridson or de Victoria (or some other)?
Lauridson is more popular nowadays, but de Victoria is one of my favorites to sing and hear. It is unusually challenging and interesting in the bass line. (Bass parts are sometimes either monotonous or jumpy, but often not melodic.)
de Victoria
The-Boys-of-St.-Pauls-Choir-School
Christmas-in-Harvard-Square
I was a coloratura soprano. The bass part was such fun that when singing it outside a choral setting, I always sang that part, too, when I could.
Our choral nun was trained in Rome and also composed. Hated pigeons. We won a number of Chicago competitions. She had a room with a cathedral ceiling and punishment for minor infractions involved cleaning her venetian blinds. I tried voice lessons with her for awhile, but she was terrifying. Had me singing while breathing at a candle. We sang bent bonelessly at the waist when we had colds. And she managed to talk our Latin teacher out of her Steinway Grand when the woman died. Good memories.