Textual: The contrast in subject in verses 16 and 17 perhaps tells us a little more of the source material that Mormon is using to write this section. Clearly in verse 17 the voice of the Lord is coming to Alma. Since it appears that this is Alma's record, we can easily understand that Alma would have recorded this information, and we have it as Alma wrote it down.
What is less clear is the source of the generalized communication from the Lord to the people in verse 16. We do not know if Mormon presumed this, or Alma wrote it down.
Given all other evidence from Mormon, it would appear that something of the communal understanding of the answer from the Lord would have been available. Alma may have written it this way, and Mormon copied it. When Mormon is inserting his opinions, (such as with King Noah) it is fairly obvious.
In this case, the most likely scenario is that Alma was emphasizing the spiritual preparation of his people. They understood that the Lord would deliver them. The specific timing, that it would happen "on the morrow" must be Alma's conclusion after he began to prepare for the journey in accord with his direct revelation.
While the people may have understood that deliverance was coming, it is doubtful that they would all have clearly understood that it would occur "on the morrow." Alma appears to be taking a small literary license here as he emphasizes the righteousness of his people.