"Sound of Music Live!", the first live musical on network television in more than 50 years and a pet project of the NBC Entertainment chairman, proved to be a worthwhile gamble on Thursday night. The three-hour event, based on the 1959 Rodgers & Hammerstein musical and starring Carrie Underwood and Stephen Moyer, reeled in a whopping 18.5 million viewers overall and delivered a 4.6 rating with viewers younger than 50. Translated from Nielsen-speak, that means more than 5.5 million people in the advertiser-coveted 18-to-49-year-old demographic tuned in. That was enough to tie CBS' top-rated "The Big Bang Theory" in the demo at 8 p.m.