The voice of God is never actually heard in the movie. The screenplay is written in such a way that the consequences of each of God's broadcasts are seen, but the broadcasts themselves are omitted.
This is apparently one of only three films in which the MGM lion is not shown roaring at the start of the opening credits, probably because of the religious theme of the film. The only other known incidence of a non-roaring lion is Ben-Hur, which also has a religious theme, and Westward the Women. (The studio's 2001: A Space Odyssey uses the illustrated lion from the MGM record label at its beginning, not a real lion, and so doesn't count.)