I don't have a Roland FP-7, but:
my guess is that the player had the keyboard "split," with one sound patch (the sound like a quartet) on the low end, and another sound patch on the high end.
At least, my Kurzweil keyboards will do that. I can set up (for instance), bass guitar sound below middle C, and piano sound above. I have several patches that produce vocal sounds, one is called "Cathedral Choir." Kurzweil sampled a vocal group named "Take five" for their vocal sounds. I frequently use that patch in our church band behind quiet slow worship songs, or as a backing for a very quiet singer.
Jim