This phrase refers to Moses. In spite of the fact that Joseph Smith is compared to Moses and that both Moses and Joseph Smith were given spokesmen, the similarity between Moses and Joseph Smith does not include this phrase about being mighty in speaking. Joseph Smith was a great speaker. The saints loved to hear his sermons, and Joseph loved to preach. The recorded sermons of Joseph Smith are powerful, colorful, and full of incredible doctrinal insights. The Lord said of Joseph, I will give unto him power to be might in testimony (DC 100:10). The testimonies of the saints regarding Joseph's sermons are as follows:
"The Prophet then arose and poured forth a golden stream of words, many of which were verily pearls without price, setting forth the restoration of the gospel and the great work that had commenced on the earth. With power he exhorted everyone who was present to seek for the truth of his and his companion's words from the source of all light, all truth, and all religion, and a knowledge of the truth of the same should surely follow." (Lydia B. Knight, They Knew the Prophet, compiled by Hyrum and Helen Andrus, p. 44)
"I heard the Prophet discourse upon the grandest of subjects. At times he was filled with the Holy Ghost, speaking as with the voice of an archangel and filled with the power of God. His whole person shone, and his face was lightened until it appeared as the whiteness of the driven snow. Finally, I was convinced of the truth sufficiently to want to be baptized, to get a knowledge for myself of the testimony that Joseph Smith had seen God." (Lorenzo Snow, They Knew the Prophet, compiled by Hyrum and Helen Andrus, p. 34)