These verses include quotes from Micah 5:8–15. Verse 12 adds the phrase “my people who” in order to include the audience in the prophecy that was given in the Old World. Micah was roughly contemporaneous to Isaiah, and his teachings would have been in the plates of brass.
There are some minor differences from Micah, the most important of which is that verses 19 and 20 are inserted, and do not appear in Micah.
In the 1830 edition of the Book of Mormon, there was a chapter break at this point, even though the sermon clearly continues. Orson Pratt combined the sermon, which is what we modern readers would expect. The original break appears to have been triggered by the end of the longer quoted passage from Micah.