Rehearsing My Choir is the musical autobiography of 83-year-old Olga Sarantos, grandmother to Matthew and Eleanor Friedberger, the creative siblings known as the Fiery Furnaces. In a most unusual collaboration, this complex story is told in fragments that range from wildly imaginative to impenetrable. Against a jagged collage of piano, organ, guitars, synthetic blips, toy instruments, and bizarre electronic effects, Mrs. Sarantos intones in a deep, slightly warbling, histrionic voice that is strangely compelling. Throughout the album she trades lyrics with Eleanor, who sings from the perspective of her grandmother as a younger woman. The vignettes follow no chronological path; it’s as if photos from every stage of Mrs. Sarantos’ life were spilled onto the floor, then picked up at random and discussed in code. There is a narrative here, but it is buried beneath dizzying lyrics that read like refrigerator-magnet poetry, such as: “zap, zapped by the zombie in the two-door Dodge/twice-baked brioche in Danish pastry pockets.” Such lines serve as clues to deciphering this sprawling, disjointed memoir. To call this possessed piece of musical theater self-indulgent is beside the point. As with most family projects, it’s designed to be insular. The commitment required to fully unravel this work (if that’s even possible) is greater than what most listeners will be willing to make, ardent Fiery Furnaces fans included. But that shouldn’t scare off the curious — the view from the outside is fascinating enough itself.