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The man who worked on arranging and composing the new music, Petar Dundakov, knows he’s about to face considerable skepticism from old fans, but he remains undaunted. “We want to broaden the sound to find a new audience,” he said. “We don’t want to stay in a museum. The deep question is, can you move folklore forward.”

Ms. Gerrard emphasized that she, and the choir’s other new collaborators, took pains not to corrupt the women’s essential sound. “We’re walking toward them, not the other way around,” she said. “If anybody is changed by this, it’s me.”

The choir actually began changing how Western listeners heard harmonies far earlier than the ’80s. In 1966, a recording from the choir, on which they performed updated arrangements of traditional folk tunes from the conductor Filip Kutev, was released on Nonesuch’s “Explorer” series and sold more than any other album on that imprint. Stars like Frank Zappa, David Crosby and Graham Nash repeatedly marveled over their vocal techniques in interviews, fueling interest in the release. Then, in 1975, the Swiss ethnomusicologist Marcel Cellier released a cassette of the choir on his own label, based on field recordings he had made since the 1950s. That’s the recording Mr. Hurwitz came across in a Parisian record store in the 1980s.

“It took about 10 seconds to realize it was something I really adored,” he said.

While he licensed the album for America, the edgy British label 4AD snapped it up for that country, where it promptly sold over 100,000 copies. Ms. Gerrard, who recorded for 4AD at the time, first heard the women with that release. “They were like lights, full of hope against any adversity,” she said. “They created a cathedral in the mouth.”