“Harmony,” a musical about a German singing group upended by the rise of Nazism, will finally open on Broadway this fall with songs by Barry Manilow and his longtime collaborator, Bruce Sussman. The show, which Manilow and Sussman have been developing for more than 25 years, tells the true story of a sextet that ran afoul of the Nazi regime because the group featured both Jewish and non-Jewish members. The ensemble was called the Comedian Harmonists. “They represent everything I love — they’re a combination of The Manhattan Transfer and the Marx Brothers, with complicated harmonies — and funny as...