It is a quirk of American theater that some of its most beloved musicals involve the specter of tyranny overseas. Fiddler on the Roof, Cabaret and The Sound of Music each serve up friendship and love in the face of vanishing personal freedoms. There are echoes of all three shows in Harmony, a musical by Barry Manilow, with book and lyrics by Bruce Sussman, receiving a beautifully staged New York premiere by the National Yiddish Theatre Folksbiene, after some 25 years of revision, delays and productions in La Jolla, Calif., Los Angeles and Atlanta.