The people's response to King Benjamin

Twice in the course of King Benjamin's sermon, the people respond "with one voice" (see Mosiah 4:2 and Mosiah 5:2). I used to wonder how they could spontaneously just cry out in unison like that when they were so many. I have later learned that it probably was not that spontaneously. We don't get all the details, but we get an interesting idea from Hugh Nibley

Hugh Nibley asks the question, "How could they cry out this long thing with one voice? The answer, as brother Nibley takes many pages to illustrate, is that these actions were part of a ceremony, the great assembly, the national assembly, the coronation of kings. In this ceremony there is a director, or a praecentor. In Greek and Roman times, he was called the stasiarch. Someone would hand him a piece of paper, the emperor would tell him, or someone else would tell him what he wanted the people to chant. He would say, "Now all together" and he would read a line and wave the flag, and they would all chant together. These were formal chants and this was the way it was done. [Hugh W. Nibley, Teachings of the Book of Mormon, Semester 1, p. 470] (Source)

What I noticed now that I haven't noticed before is how the people both times echo the words of King Benjamin in their response. Consider this:

2 And they had viewed themselves in their own carnal state, even less than the dust of the earth. And they all cried aloud with one voice, saying: O have mercy, and apply the atoning blood of Christ that we may receive forgiveness of our sins, and our hearts may be purified; for we believe in Jesus Christ, the Son of God, who created heaven and earth, and all things; who shall come down among the children of men. (Mosiah 4)

They are using the same expressions that King Benjamin used

Ye cannot say that ye are even as much as the dust of the earth (Mosiah 2:25)

the atoning blood of Christ (Mosiah 3:18)

Jesus Christ, the Son of God, the Father of heaven and earth, the Creator of all things (Mosiah 3:8) 

shall come down from heaven among the children of men (Mosiah 3:5)

The same thing is happening when they "cried out with one voice" again in Mosiah 5:2-5

2 And they all cried with one voice, saying: Yea, we believe all the words which thou hast spoken unto us; and also, we know of their surety and truth, because of the Spirit of the Lord Omnipotent, which has wrought a mighty change in us, or in our hearts, that we have no more disposition to do evil, but to do good continually.

3 And we, ourselves, also, through the infinite goodness of God, and the manifestations of his Spirit, have great views of that which is to come; and were it expedient, we could prophesy of all things.

4 And it is the faith which we have had on the things which our king has spoken unto us that has brought us to this great knowledge, whereby we do rejoice with such exceedingly great joy.

5 And we are willing to enter into a covenant with our God to do his will, and to be obedient to his commandments in all things that he shall command us, all the remainder of our days, that we may not bring upon ourselves a never-ending torment, as has been spoken by the angel, that we may not drink out of the cup of the wrath of God.

Compare with

Lord Omnipotent (Mosiah 3:5, 17, 18)

goodness of God (Mosiah 4:5, 6)

exceedingly great joy (Mosiah 4:11, 20)

And the things which I shall tell you are made known unto me by an angel from God... endless torment, from whence they can no more return; therefore they have drunk damnation to their own souls. Therefore, they have drunk out of the cup of the wrath of God (Mosiah 3:2, 25-26)

"Goodness of God" and "exceedingly great joy" are not unique expressions in the Book of Mormon, but since they are repeated several times in these chapters, I think the reference is intentional. "Lord Omnipotent" is not found anywhere in the Book of Mormon except for King Benjamin's speech.

So what does this mean? Does this mean the people have been paying so close attention that they can remember several of the exact expressions King Benjamin used after hearing it only once? Probably not. Rather, it means that it had already been written down

therefore he caused that the words which he spake should be written and sent forth among those that were not under the sound of his voice, that they might also receive his words. (Mosiah 2:8)

So they already had the written word from King Benjamin and could base their response on that. If Hugh Nibley is right, this response was written down too for a director who could lead the chant. The textual connections support this idea of a pre-written chant as a formal ceremony, which in turn explains how they could all "cry out with one voice". They do not support the idea that Joseph Smith, with his face in a hat and without access to notes or manuscript, made this all up.