“In honor of its 50th Anniversary,” a copywriter for the Web site says, “the Playhouse is proud to present this unsung sensation in its full, original, and soaring orchestration.” Unsung sensation? Actually, A Little Night Music has been a critical and popular favorite since its premiere in 1973. And since Judy Collins blazed a trail with her cover of “Send In The Clowns,” recording artists beyond number have put forward versions of their own. (Sondheim frequently called the song his only hit.) One among the fans of Sondheim’s most Mozartean confection (note the title) was the filmmaker Ingmar Bergman, whose comedy Smiles of A Summer Night inspired it. Though best known for his explorations of the darker corners of the human psyche, the Swedish legend had his lighter side, too. When Bergman took in A Little Night Music, he was in New York hoping to engage Sondheim to write English lyrics for a new adaptation of Franz Lehár’s Silver Age operetta The Merry Widow. ’Twas not to be. (Now there’s a project you might call “unsung.”) —Matthew Gurewitsch