*sigh* Alright, time to set the record straight. I not only happen to be a plainchant afficionado, I sing plainchant fairly frequently, in church settings as well as at home.
For starters, the atmosphere is NOT sad at all. Plainchant has been written in a way that even the *key* would suggest wonder, joy or sadness. This plainchant that you sampled is not even sad. It evokes joy and wonder. That background you added doesn't make it sad either, it just.... spoils it.
That bassline doesn't switch anywhere past C and D. There's no variety in the bassline or even the chords, which makes it sound dull and flat.
I hear what sounds like 'Qui manducat' in the centre, i.e. around 1:45, but because it's in a completely different key to what you've got, it sours the piece even more.
You've got alright mixing -- in fact, everything seems to be well-placed between the headphones, but I don't hear much volume control for the individual instrument. There's no loud-soft; it's basically the same loudness throughout. Another minus point for this piece, because it makes it dull and flat.
Now, that you did this to a plainchant that evokes joy and wonder ALREADY ruins it for me. I've uploaded two plainchant pieces to NG myself: Qui manducat, and A Solis Ortus Cardine. The way it's sung and the key it's written in can tell a story, even if you don't understand the lyrics. Now if the chants you used were coupled with, say, orchestral strings to bring out the grandeur of the chant, great! But no.... just no.
SUGGESTIONS:
1) Practise some form of volume control for your instruments, as well as for your full piece. You want some loud-soft. You want some change of atmosphere, whether it be an ambient piece or otherwise. Peaks and valleys, highs and lows. That's what makes a great piece.
2) Look up plainchant styles before you decide to pick one apart. This one JUST DOESN'T SOUND SAD AT ALL. It just sounds completely ruined. :/ Study the lyrics, see what feel they give off, hear the key and the notes, and then see what purpose each plainchant piece was written for.
2/5.