Mannes Opera's production of "The Silent Serenade" now seems retro with its lilting score, a combination of Johann Strauss II and Jerome Kern who was working in Hollywood during Korngold's tenure. This may be why the frivolous plot seems like that of a Fred Astaire/Ginger Rogers movie. (Ironically, Korngold's romantic operas sound Puccini-esque and his orchestral film scores found influenced by Wagner.) While the lib…