Production value stuff I won't really comment on, since all things considered you're probably a better sound designer than I am. @mark styles has provided good information to you regarding some engineering stuff.
There are some parts that overlay in diatonically polytonal language that confuse me sometimes. Around the 2:30 mark, you retain your i - iv vamp but your melody goes to a iv - v vamp, which creates a im11 and a iv13(#11) sound which can be distracting. I know the point is to retain to lines at once, but sometimes it doesn't work for me.
Horns playing high C at the beginning is a difficult thing for them to do, the highest of most player's tessituras. Careful writing concert F's.
The sections are quite nicely flowed through, though when it comes back down to the chants, it never seems to leave that portion of the dramatic arc, leaving the ending feeling rushed.