The people hear the voice speaking, but they don’t understand its words until the third time. This has other parallels in scripture; in the Old Testament/Hebrew Bible, for example, Eli does not understand that it is God’s voice calling Samuel until the third time the boy tells Eli that he heard a voice calling his name (1 Samuel 3:1–9).
And in Joseph Smith’s account of the coming forth of the Book of Mormon (now canonized as part of the LDS book of scripture called the Pearl of Great Price), the angel Moroni tells Joseph the same information about the golden plates’ history and significance several times with almost no variation.
It would seem that people need time to adjust to hearing divine voices and receiving angelic visitations; they are so overwhelmed at first that they are liable to miss the all-important spiritual message that is to be imparted. God and his messengers repeat their teachings so that people will have a chance to attend carefully to the words.