I would love to see a sound set around focused around religious rituals/ceremonies.
I think these would be pretty cool:
-Hymns from a choir
-A sermon in a mystery language (Angry/Happy/Neutral in tone)
-Melodic Chanting
-Call and response sermon (Single person says something then followers chant in response)
These could be both creepy and non-creepy in nature.
There are some like these already (though not all):
In the Fantasy Player - Bell Monk Temple and Temple of the Jade Dragon have some chanting, meditation sounds. Elven Vale Night has a single singer doing a meditative melodic song (vaguely hymn-esque). The Temple of Razmir has some religious declarations/mini sermons and temple bells.
The Sci-Fi player has a Cultists set with chanting and speaking in an unfamiliar language. Also braziers, and the screaming of a victim about to be sacrificed, so a good rite for your players to interrupt!