Brian C. Hales notes that there are 68 sermons on dozens of topics in the Book of Mormon. Some of these sermons, including that of King Benjamin, are quite long and full of deep spiritual content. Richard Bushman rightly points out that:
He [Joseph Smith] is not known to have preached a sermon before the Church is organized in 1830. He had no reputation as a preacher.– Richard L. Bushman, “A Historian’s Perspective of Joseph Smith,” on Joseph Smith’s Relationship with God (Salt Lake City: Covenant Communications, 2007), CD2, track 8.
How could Joseph Smith dictate 68 sermons on the fly if he had never given a sermon before?