Nephi uses Isaiah 29:4 as the basis for this expansion. Our verse 16 is mostly a quotation, but Nephi adds, after quoting that “their voice shall be as one that hath a familiar spirit,” the idea that the Lord will give those from the dust power to speak to men—to “whisper concerning them.” The important aspect is that it would come from out of the dust. It appears that Nephi’s vision included hiding the Nephite record in the ground, which would be a more literal fulfillment of this prophecy that even Isaiah would have understood.
At this point Nephi departs from Isaiah and continues the way he applies Isaiah to the future of his people’s writings. Those words are sealed up in a book. It will not be available to those descendants of Nephi who have dwindled in unbelief.