It was a convent at a crossroads in 13th century Spain, and into it flowed not just pilgrims on the way to Santiago de Compostela, but music. And if there was once doubt about whether the sisters sang that music themselves, the four women of Anonymous 4 are singing it now.

In fact, they’re singing the sisters’ rich polyphonic music and sacred Latin songs Sunday, November 14 at St. Andrew’s Episcopal Church, where the Academy of Early Music presents them in a 4 p.m. concert, “Secret Voices: The Sisters of Las Huelgas.”

The concert, said Susan Hellauer, a founder and one of this stellar a cappella vocal group’s members (along with Ruth Cunningham, Marsha Genensky and Jacqueline Horner-Kwiatek), is “one of those things that has been on my mind since the very beginning.”

That’s nearly 25 years and 18 CD’s ago (The group’s most recent album, “The Cherry Tree,” a Christmas recording, was issued in September).

The music was unearthed by the Monks of Silos, in Spain, at the beginning of the 20th century. “It was a real treasure trove, and it’s been in the eyes and microscope of scholars since then,” Hellauer said.

But Hellauer, a scholar of Medieval music herself, and the members of Anonymous 4 didn’t want to be pegged as a women’s music group when they first formed — “That was anathema to us,” she said — so they put the project aside, just as they waited 10 years before working on the music of Hildegarde von Bingen.

“But the Las Huelgas Codex did provide evidence that the sisters — wealthy and aristocratic women who were members of the Cistercian order — actually sang the music themselves.

“Some said the nuns sang it, while others said, no, they hired male singers,” Hellauer noted. “Singing polyphony (was) very much against the rules, but the sisters were passionate in their devotion, and there were members of the royal family and the aristocracy in this convent. The bishop of monastic houses was always on their case for breaking the rules,”

One of the pieces in the Codex is a unique vocal “solfeggio” exercise for the sisters, in which they practiced singing their hexachords under the watchful ear of the music mistress — proof of the sisters’ dedication to music that Hellauer describes as “a European tour of the virtuoso polyphonic music of the time from 1200 to 1300.”

When a new facsimile of the Codex came out, Hellauer was thrilled — “The existing transcriptions were not satisfactory to us,” she said — and correspondence with British scholar Nicholas Bell, an expert on the Codex, convinced her she was on the right track regarded the notation.

Carrying out the ideas by singing the music was the next logical step. Hellauer said it is not hard to fathom how the music, which the sisters retrofitted with new lyrics that made devotional pieces of former love songs, for example, came to the sisters’ convent.

Burgos, where their convent was located, was a major stop on the pilgrim route to Santiago de Compostela, and visitors would have been frequent. The sisters, she said, had the money, power and influence to have things brought to them from across Europe.

“It’s hard for us to imagine how pieces could become real hits” all across Europe at that time, she said, but people — and music — traveled widely in the 13th century, albeit along roads that were sometimes little more than donkey tracks. So whether pilgrims left the sisters music as a token of thanks for hospitality or whether they sent emissaries to seek the music out, the international nature of the music is not unexpected.

But it makes for welcome listening and wonderful singing, a fascinating look inside convent walls long ago.

Susan Isaacs Nisbett is a free-lance writer who covers classical music and dance for AnnArbor.com.

Anonymous 4 performing live on New York Public Radio last year: