As part of the important testimonies of this spectacular and unprecedented event, Nephi recorded that all the people witnessed Yahweh’s first address to them (v. 1); now they witness, or testify, to the second occasion.
While the text indicates that “all” heard the voice, that term must be localized to Bountiful. Those are the people of whom the story is being told. It is to them that the Messiah comes. “All” would be all in Bountiful, not all of the Americas, or even all of the former lands of Zarahemla.