Even though they were in the announcement trailer 
I have a sad now :â(
Maybe theyâre holding back or something. Maybe theyâre trying to surprise us or something, and weâll hear it in some climactic moment of the campaign.
We havenât seen all the music yet.
Remember that not all music in the game gets put on the soundtrack. For all we know it could be in the main menu. I say wait until the game is released (or leaked) until we make that assumption.
If you mean âClassic Monksâ as in the old Halo theme 343 has said from the beginning that they will be making all their own music and will be using little to none form the past games. If you mean chanting monks in general, âRevivalâ would like to have a word with you.
Nah, theyâre saving the classic monks and classic main theme for their last appearances until the most epic of moments: the final events of Halo 6, and the end of an era 
Ending it with the music that began it all would be the only way to conclude the Halo saga with due justice. Calling it right now haha.
> Even though they were in the announcement trailer
So what youâre telling me is youâve heard 100% of the gameâs sound track!?
I was wondering the same thing OP, perhaps the announcement music is just not being officially announced yet? Or I hopeâŚand yeah my favorite song right now is revival JUST because of the monk type chant,
Here is what Iâm thinking about this issueâŚ
In many videos and certain (but very few) soundtrack pieces you here bits of old Halo music. Like someone else already stated the soundtrack doesnât contain all the music from the game. I think 343 wanted to make sure all the ânew and uniqueâ music makes it to the soundtrack. While the more similar to old Halo soundtracks just saved for the gameâŚ
Here are some examples of music that sounds similar to previous tracks.
Similar to âOne Final Effortâ and other various Halo tunes.
Similar to âBehold a Pale Horseâ(and all the other versions of the âhorseâ song).