This one's about haunting musical traditions in Appalachia. It ought to occupy you for a while.
The name "Sacred Harp Singing" comes from a hymnal written in New England in 1844, but the tradition goes back to the 18th century. The musical notation is called "shaped note" and was invented as a simple way for people who did not know standard musical notation to sing hymns by recognizing the four different shapes of notes. Traveling preachers sold the Sacred Harp hymnals in the hills and hollers of Appalachia, where things tend to survive.
The leader stands in the middle of the congregation, which is divided into four sections, and conducts. The documentary shows an eight-year-old girl as leader.
Sacred Harp singing almost died out. It got a boost from the horribly depressing 2003 Civil War movie Cold Mountain. The style can also be found in the British Isles, sometimes called Gaelic Psalm Singing. The Bear's slapdash research seems to show it was taken across the Atlantic in the 19th century and somehow adopted by Scottish Presbyterians in remote places.
Fortunately, Sacred Harp singing is making a comeback, as recounted in the second clip.
EDIT: A kind person Tweeted this link on Sacred Harp music. It includes many local groups. (If you don't follow the Bear on Twitter, he's @CorbiniansBear also linked on the sidebar. His Twitter account and official Facebook page are excellent ways to know when there's a new post here and enjoy random misfirings of his 450 gm brain found nowhere else.)
All these tunes are spine-tingling for the Bear, whether Sacred Harp, lined-out hymnody, or traditional mountain folk. He has culled YouTube for a variety of clips for your edification and listening pleasure. (Movie links are to Wikipedia, whose plot summaries include spoilers.)
The first clip, "Good Old Way," is 2:36 long and for the ears only. If you listen carefully, you can hear the singers begin practicing the tune in the four shaped notes. That way, the congregation had the tune down before beginning the actual hymn.
The second clip is a seven minute long documentary that includes the history and revival and is well worth watching.
The third clip is not Sacred Harp, but lined-out hymnody running a hypnotic 3:29. This form of call-and-response music is usually associated with black churches, but it was also used by illiterate white folk in the mountains of east Tennessee and West Virginia.
The fourth clip is a four minute departure to a wonderful scene from the Coen Brothers great 2000 movie, O Brother Where Art Thou. It is loosely based on Homer's Odyssey, and the seductive ladies are sirens. (Caution: PG Sensuality - they're sirens!) Gillian Welch, Allison Krauss and Emmylou Harris are the voices behind the actresses. This is one of Bear's favorite movies and showcases George Clooney's comic chops as well as some great music, including the "Po Lazarus" chain gang song by the a capella gospel Fairfield Four. (Bear's daughter Ragsy loves this movie, too.)
Finally, there's a haunting and edifying song from the 2000 film Songcatcher, "O Death" running 2:37. In an interesting twist, an unlikely character proves he has not forgotten his roots by singing a verse, then other characters each take successive verses. It's a good movie about a woman who heads into the hills in 1907 to record traditional Appalachian music. Young Emmy Rossum shows talent far beyond her years. Watch the full movie for the music, but beware of the lesbian scene out of left field at the very end. (Sorry, Land Shark, there is no lesbian scene at the end of the clip Bear chose.) It's worth watching up until that - just stop when you see the objectionable scene coming and read the ending on Wikipedia.
Sacred Harp Singing: "That Good Old Way"
The Denson Parris Sacred Harp Singers
The Denson Parris Sacred Harp Singers
Sacred Harp Singing: Documentary
Pop Goes the Culture
Lined-Out Hymnody: "I'm Going to a City"
Indian Bottom Association of Regular Baptists
O Brother Where Art Thou: "Go to Sleep Little Baby"
Coens, Touchstone Pictures / Universal Pictures
Gillian Welch, Allison Krauss, Emmylou Harris
Songcatcher: "O Death"
Written / Directed Maggie Greenwald
Songcatcher LLC / distributed by Lions Gate Films