Oh dear, I was hoping there was going to be one blindingly obvious answer, because I'm going to struggle to be more specific. I can quite clearly 'hear' the sound in my head but I'm not sure how to describe it...
There were no instruments, as far as I can recall.
Mixed voices (mixed pitches, though I couldn't swear it was mixed male and female).
Speed... sort of a fast-ish speech tempo, as if you were saying "ba-da-ba-da-ba-da-ba" in normal speech. Not long, drawn-out notes like a Gregorian chant or something, just short sharp syllables: "ba-da-ba-da-ba-da-ba"...
Sorry, I know that's not very useful