Well, I always felt that (Especially with the music in Keen 7) that this was kind of a dark turn for the series. I wanted to keep some of the cheesyness of keen music, hence the little jazzy bits, but I did want to give a feeling of urgency and some dread about what could happen if Keen fails.
My "Sad 'Shrooms" song (I think I might go with that) was made with that specific level in mind (Forest of Mushrooms) but I think it also gets across the feelings I mentioned.
I wrote the music for Nospike's mod, which is definitely more cheery and upbeat, so I wrote music to reflect that: http://www.youtube.com/watch?v=QDla-P1U2Sk
But like I said, I was always under the impression that Keen 7 should be darker and not have as much cheery music, but I dunno.
Monastery of Mystery sounds more like a level name, actually, lol.