YOU HAVE BEEN WARNED Remember those Rocky Horror Picture Show sing-alongs, where folks went to the movie dressed up as Brad and Janet? Starting its tour in Britain this week, and planning to hit American shores shortly after, is the Sound of Music Sing-a-Long-a, where moviegoers arrive in nun outfits, in lederhosen or as the whole family Von Trapp. Singing along (the songs are subtitled) is encouraged. Dancing, booing Nazis and whistling are optional. Foam nuns are available in the lobby.