Mormon indicates that he has made an editorial decision. That decision was to stop at this place, and not include other words of Alma to the people. Mormon does not indicate whether or not there were other words on this occasion, or whether there were different occasions. He simply tells us that he has elected to limit his inclusion to these discourses.
It may be presumed that had Alma spoken to a different group of Ammonihahites that the message would be similar, so perhaps Mormon didn't include other sermons because they substantially repeated the information in this discourse. This particular discourse not only provides doctrinal information that would be pleasing to Mormon, but the confrontation with Zeezrom leads directly to the next narrative section. We may therefore understand that this particular set of sermons was selected for the story of Zeezrom as much as for the doctrine within them. If Mormon has selected this information particularly for the story of Zeezrom, we should pay attention to that story, as Mormon considers it important for us.
Textual: There is no chapter break in the 1830 edition. For Mormon, this sermon is a continuation of the setup for the story of Zeezrom's conversion. Alma has just given a call to repentance, and Zeezrom is a lawyer in need of repentance. Mormon's inclusion of this call to repentance serves as an introduction to the repentance of Zeezrom. Since this is Zeezrom's story, it is both a new chapter at the beginning of this discourse, and a continuation at this point. Mormon made no break here because he was just now getting to the point of his original chapter selection.