Yes, I know about sacred harp singing. I think there was a special on it on the History Channel or PBS several years ago. It has a very unusual sound, but is very beautiful.
When I was a child, I use to go to all day sacred harp singing. I loved to listen to it, but I never learned how to sing it. I remember going to one at the old church that my mom went to as a child. The best part about it was a wash tub full of the best lemonade I have ever had. Well, maybe being thirsty helped the taste a little bit. This was before many churches were air conditioned except for the hand held fans. I went with my mom's aunt.