LOS ANGELES (Reuters) - Country singer Carrie Underwood will sing the opening theme to NBC's "Sunday Night Football" program, one of the United States' most-watched TV shows, when the new season kicks off in September, the network said on Tuesday. Grammy-winner Underwood will take the reins from fellow country singer Faith Hill, who had sung the theme "Waiting All Day for Sunday Night" before each telecast since 2007. Underwood's performance will be prerecorded. "I'm very excited," Underwood, 30, said in a conference call. ...