Florence Foster Jenkins, one of the world’s most famous society eccentrics, suffered under the delusion that she was a great coloratura soprano when she was in fact incapable of producing two consecutive notes in tune. Nevertheless, her annual recitals in the ballroom of the Ritz Carlton hotel, where she resided, brought her extraordinary fame. As news of her terrible singing spread, so did her celebrity. The climax of her career was a single concert at Carnegie Hall in 1944. Famously, it sold out in two hours. “Souvenir,” by turn hilarious and poignant, tells her story through the eyes of her accompanist, Cosme McMoon. A talented musician, he regards her at first as little more than an easy way to pay the rent, but, as he gets to know her, his initial contempt gives way to reluctant admiration, then friendship and affection.