This phrase forms a bracket on the experience with the Savior. The account we have not only must have been crafted after the events because of the widespread nature of the events described, but they are also presented in a stylistic form that indicates careful construction.
Notice how the experiences with the voice of the Lord are bracketed here. Just before they hear the world of the Lord, we have:
3 Nephi 8:23
23 And it came to pass that it did last for the space of three days that there was no light seen; and there was great mourning and howling and weeping among all the people continually; yea, great were the groanings of the people, because of the darkness and the great destruction which had come upon them.
3 Nephi 8:24
24 And in one place they were heard to cry, saying: O that we had repented before this great and terrible day, and then would our brethren have been spared, and they would not have been burned in that great city Zarahemla.
3 Nephi 8:25
25 And in another place they were heard to cry and mourn, saying: O that we had repented before this great and terrible day, and had not killed and stoned the prophets, and cast them out; then would our mothers and our fair daughters, and our children have been spared, and not have been buried up in that great city Moronihah. And thus were the howlings of the people great and terrible.
The essential features of the introduction to the Lord’s statements are:
These will contrast with the quiet between the two utterances that came on this occasion.
The ending bracket comes from verses 8 and 9 here.
Of course the people would still be in mourning, and it is understandable that they would return to mourning, but it is quite unlikely that the return happened immediately after the proclamation of the Lord. They had waited in silence for a space of time between the two utterances, and they would likely have remained as still after this one. The final bracketing of the return to mourning is the author’s way of telling us that this event has now ended. We have no more voice of the Messiah from this incident.