A rock opera, when done right, is a thing of beauty. The Who did it with Tommy, David Bowie with Diamond Dogs, and, arguably, the Fiery Furnaces did it with their last album, Blueberry Boat. That album had spunk — with its water-logged piano and acid-drip distortion, it came across as a grand seafaring adventure. But in the wake of Blueberry Boat’s success, the Fiery Furnaces don’t quite make it with Rehearsing My Choir. It features the lyrical stylings of Mrs. Olga Sarantos, the 83-year-old grandmother of Furnaces Eleanor and Matt Friedberger, and is based loosely on her memories, which are mostly about marriage. Lyrically, the songs are interesting enough, and Olga’s delivery comes across as stodgy, in a good old-person sort of way, but the narratives get tedious a few songs in. The album’s construction is squarely in the abstract, often lacking rhythmic balance; though not always a comfortable feel, this arrhythmia adds quirky texture to the rickety backing piano. Taken as a whole, the album plays like a hallucinogen-induced bedtime story told by Kermit the Frog, making for an interesting and ambitious, if not wholly palatable, offering.