O great mystery, and wonderful sacrament, that the animals should see the new-born Lord lying in a manger. Blessed is the Virgin whose womb was worthy to bear Christ the Lord. Alleluia
The piece is not all that difficult. Before you dismiss that claim, a story. When our schola was founded, we all looked forward to a time when we could sing this. It seemed impossibly difficult.
Years went by. We worked a bit each Christmas season and still never got it together. I think our first time to sing it was three years ago. Now we pull it out every year without even thinking much about it, and play with rhythms, tempos, articulations, and effects. It is just a normal part of our repertoire.
I tell the story to encourage scholas not to become discouraged and always think about the future. Improvement comes over time, and one day you will turn around and everything will fall into place.
I'm posting this performance from Taiwan, just because I found it beautiful and charming. There are many versions on youtube, and I encourage you to listen to them all, and see how flexible the piece is. It sounds different with different forces and tempos. You can say, oh, this one is too fast, or, oh, this one is too slow, but in the end, this robust music accommodates many senses of how it ought to be.
(By way of contrast, consider this darkened version with instruments, with that magic touch of Jordi Savall.)