F rom behind a closed door, and to the untrained ear of a reporter, it sounds like The Omen soundtrack. In fact, the Island Community Chorus is rehearsing an aria from Felix Mendelssohn’s Elijah, the most ambitious piece director Peter Boak’s choir has attempted in its 11-year history.

“The fire descends from heaven!” calls the director. The assembled hundred-or-so altos, basses and sopranos, blast the line around the back room of the Performing Arts Centre.

“Okay: one, two and breath,” coaches Mr. Boak, addressing the semi-circle of singers from his stand as they consult their 194-page manuscripts, many of them heavily annotated. “You have to make the fire descend from heaven here. Sit up and show your face. Let’s go from the top...”

With its arias, solos, cast of characters and orchestra, Mendelssohn’s complex three-hour work is the definitive oratorio of the Romantic period.

“I’ve never conducted on this scale,” says Mr. Boak, who also teaches music at the Tisbury school. He will conduct the chorus as well as an orchestra, simultaneously, for this performance.

It is just after Monday’s two-hour rehearsal, the second last practice-run ahead of Sunday’s performance, which will be performed in the high school’s Performing Arts Centre. “This is the most demanding, largest reading we’ve done. An oratorio has everything that an opera has without the staging,” he says.

The choir has met for 13 two-hour weekly rehearsals since January, as well as a few Saturday sessions, and many have practised at home using an online tutoring program.

Powerful second soprano Binnie Ravitch’s manuscript is decorated with multicolored tabs and scuffed by months of close attention. Previously a senior corporate manager at Citigroup, Ms. Ravitch, who moved here after retiring last year, started out as a professional actress.

“It’s harder than learning lines for plays,” she said, during a break. “As an actor, timing and melody are up to you.”

The challenge of acting out a story while sitting in your seat and singing as a group, is a juggling act, she said. “And it’s vocally demanding, which some of us are still working on.”

Judy Crawford, director of the board for the chorus and an alto in the group, is a veteran choral singer.

“I was in a choir before I could even read,” she said. “Peter is by far the best director I have served under.”

Mr. Boak does not hold auditions and encourages first-timers to mix with singers holding music degrees.

“There’s bankers, lawyers, dancers, retired teachers and nurses,” said Mrs. Crawford. “They all love to sing, that’s the common denominator.”

Over the years, the close-knit group has become an integral part of the lives of long-term members.

“Ask anyone, Monday is the most important night of the week,” she said. “Everyone works so hard to not miss a rehearsal. It’s like a family with 120 members.”

Though a single private donor has provided the group with a sizable donation for the past three years, it relies on voluntary contributions at their events to cover costs.

The economic constraints of the volunteer organization, however, have not limited Mr. Boak’s ambition on this project: the bulk of the show’s 24-piece orchestra is cherry-picked from Boston’s professional performers. As for soloists. Mr. Boak recently spotted Aaron Guckian at a Cape Cod Opera production, a bass baritone soloist (he moonlights as an advance man for the Rhode Island governor, Donald Carcieri) who will sing the part of Elijah.

Chappy resident Abigail Southard will perform the soprano solo. Tenor Mark Nemeskal and alto Diane Rodwell both will travel to the Vineyard for the performance.

“They have had lengthy singing careers,” said Mr. Boak, who has also arranged for an organ to be shipped to the Island for the performance. “I pulled a lot of strings, let me tell you,” he added.

The singers will have a single full rehearsal with orchestra tomorrow before their one-night-only performance on Sunday. Though there is a certain skittishness among the performers, judging by several concerns raised by singers at Monday night’s rehearsal, Mr. Boak was not worried.

“I don’t really panic,” he said. “This choir hasn’t laid any eggs, yet. I hope this will be a star in our crown.”