At Sacred Harp singings, the country churches of the American South resound with the harmonious pulse of song. There are no instruments, just human voices carrying on in four-part, a cappella harmony. Loud and hypnotic, it doesn’t sound like a traditional, melodious sing-a-long. Rather, it’s like a swell of voices chanting with such raw emotion, it sounds as if the room might burst. In this style of music, the tenor, alto, and bass are independent of the melody. With the four parts so distinct, they’re printed on separate staves. This…