Rhetorical: In the context of these last verses, Alma's question about their continued commitment to the gospel involves more than just a possible temporary lapse. In this context, Alma is suggesting that there are those who may have been truly converted, but who have since fallen. When he asks "can ye feel so now?" Alma is asking whether or not they have committed any of these sins of which he has been speaking. He is pointing his question at every person in the congregation and asking them to examine themselves and their actions.
Spiritual: We need no assume that this question must be pointed at those who sin gravely. It is quite possible that all who have at one time sung "the song of redeeming love" may have become complacent, and no longer have present in our hearts that same passion for the gospel. Alma is not only commanding introspection, but encouraging us that it is possible that we might continue to feel that way with God. The way, of course, is to follow the gospel, and do those things which are pleasing to God.
Translation: Joseph Smith used the available vocabulary of his day to express the concepts that Alma is describing. Such phrases as the "mighty change" and the "song of redeeming love" were common phrases in the religious revival camps with which Joseph was familiar (see Thomas, Mark D. Digging in Cumorah. Signature Books, 1999, 132-4). The presence of this particular phraseology is no more surprising than the clear usage of New Testament phrasings in the Book of Mormon which took place prior to the New Testament times. It indicates no more than that Joseph used models of speech in his translation with which he was familiar.