The hills are alive with the sound of Julie Andrews. Sing-along versions of the perky perennials The Sound of Music and Mary Poppins are suddenly everyone’s favorite things. Sing-A-Long Sound of Music opened last year in Britain, and trots into Gotham’s Ziegfeld Theatre Sept. 6 (a national tour is planned for 2001). ”It’s like a combination of being at church and being at a British soccer match,” cracks Sing-A-Long coproducer David J. Foster, who says the night’s best moment occurs during that Alpine-dervish opening shot. ”The whole audience puts their arms in the air and bursts into song, and it really is just incredible.” Especially given the crowd comes dressed as Nazis and nuns — and the occasional lonely goatherd. In L.A., the supercalifragilistic nanny got the Rocky Horror Picture Show treatment with viewers adding their spoonful of sugar to a Disney-sponsored run of Mary Poppins. According to Lylle Breier, Disney’s senior VP of special events, the show’s success means more musical merriment — Disney is combing its library for the next installment. Yodelayheehoo!