The words of this speech are sufficiently beautiful that they have been set to inspirational music. The context is the meeting between Alma and the sons of Mosiah (see vv. 14–17). This speech appears to be Alma’s response to Ammon’s joyful testimony (Alma 26). It is important for the context of this sermon that we remember these friends’ meeting:
And it came to pass that as Ammon was going forth into the land, that he and his brethren met Alma, over in the place of which has been spoken; and behold, this was a joyful meeting.
Now the joy of Ammon was so great even that he was full; yea, he was swallowed up in the joy of his God, even to the exhausting of his strength; and he fell again to the earth.
Now was not this exceeding joy? Behold, this is joy which none receiveth save it be the truly penitent and humble seeker of happiness.
Now the joy of Alma in meeting his brethren was truly great, and also the joy of Aaron, of Omner, and Himni; but behold their joy was not that to exceed their strength. (Alma 27:16–19)
Mormon does not specifically mention Alma’s speech, but the situation so closely resembles details in this discourse that I think we may, with some assurance, assign this speech to that time.
Redaction: This quoted soliloquy, while eloquent, interrupts Mormon’s narrative. After inserting this speech, he must return to the narrative thread again. (See commentary accompanying Alma 30:1.)
Since the rest of the material appears to come from Ammon’s records, the placement of this speech and the speaker indicates a change in source material. It is much more likely that Mormon found this discourse in Alma’s record than in Ammon’s. Mormon provides no introduction to the speech itself after concluding the story of the mission of the sons of Mosiah. Usually Mormon creates a new chapter when he switches sources or quotes a speech. In this case, he does not. Alma’s soliloquy clearly belongs with the mission story, but the abruptness of its inclusion strongly suggests that he is quoting from a different source. I speculate that, when he finished the account of the sons of Mosiah and returned to Alma’s sources, he found this beautiful sermon and decided to include it.