Earl Nightingale, 68, voice of Sky King, “America’s favorite flying cowboy,” during the 1950s. The gravel-voiced broadcaster, who was the second voice of the pilot of the Songbird as it flew through hundreds of adventures, was also a pioneer in the production of motivational audiotapes. After serving in World War II, where he was one of a handful of Marines to survive the Pearl Harbor attack on the battleship Arizona, he began a daily talk show, “Our Changing World,” on radio station WGN in Chicago. It went into syndication in 1949 and is still carried across the nation and in a number of countries overseas. From 1950 to 1954, Nightingale was the radio voice of the “Sky King” adventure about a rancher who used an airplane instead of a horse to fight bad guys. (Roy Engel was the original hero of the daily serial.) In 1960, Nightingale and Lloyd Conant formed a company that produces tapes by motivational speakers, including Chrysler Corp. Chairman Lee Iacocca. In Scottsdale Valley, Ariz., on Saturday of complications after heart surgery.