Once upon a time, Americans grew up humming show tunes. They dominated radio airwaves, so, even if you hadn’t seen the musicals (or the films made from them), you knew the melodies and words to the songs of “Oklahoma,” “Carousel” and “My Fair Lady.”
But that changed as pop, rock and rap started to control airtime. Today, it’s a rare Broadway song that cracks the national consciousness. It’s in part for this reason that James Lapine’s HBO documentary, “Six by Sondheim,” which debuts December 9, is so fascinating and important.