Mormon now appears personally in his record. Introducing himself, he twice tells us that he is "sober." The context is that Ammaron is handing on the records to him, by telling young Mormon where he'll find them when he's grown. Ammaron calls Mormon sober, and later Mormon identifies himself in the same way.
The word "sober" only appears twice elsewhere in the Book of Mormon. Those instances are close together, and share a context with each other and with Mormon's sobriety: at the end of Alma II's career, as he commissions his son Helaman to be the next recordkeeper, he urges both Helaman and Shiblon to be "sober."