According to Sidney Sperry, literal translations of Hebrew idioms are prevalent in the Book of Mormon. Parallels can be found in the Old and New Testaments, especially in the Hebrew translation of the Old Testament. . . . The common Hebraic idiom "to open the mouth" appears in the Book of Mormon. One of these occurrences is in Alma 18:18, "King Lamoni did open his mouth, and said unto him."
This manner of writing is somewhat foreign to us, but occurs in the Old Testament as these examples attest:
After this opened Job his mouth and cursed his day. (Job 3:1)
Then I opened my mouth and spake. (Daniel 10:16)
[Sidney B. Sperry, "Hebrew Idioms in the Book of Mormon," in Journal of Book of Mormon Studies, Vol. 4/1 1995, pp. 218, 222]