Isaiah 52:8 (King James Bible) thy watchmen shall lift up the voice with the voice together shall they sing
While copying from 𝓞 into 𝓟, Oliver Cowdery initially dropped the phrase “with the voice”, probably because his eye skipped from the immediately preceding voice in “lift up the voice” to the voice at the end of “with the voice”. Oliver later inserted “with the voice” with somewhat heavier ink flow, using a sharper quill (which argues that these supralinear words were inserted later, probably when Oliver proofed 𝓟 against 𝓞). The corresponding passage in the King James Bible has this phrase; undoubtedly the original manuscript had the phrase, in agreement with the biblical text. It should be noted that there is no evidence that Oliver ever used a King James Bible to correct the text in the manuscripts. Consider especially the many instances from the Isaiah quotations where the Book of Mormon text is missing a phrase or a clause that originally occurred in the King James text (for 15 examples, see under 2 Nephi 8:15). On the other hand, there is clear evidence that the 1830 typesetter did occasionally refer to a King James Bible to supply or correct a word or two when the manuscript reading seemed impossible (but never one of these missing phrases or clauses); for examples, see the discussion under 2 Nephi 17:23 and 2 Nephi 20:10.
Summary: Retain in Mosiah 12:12 “with the voice”, Oliver Cowdery’s correction in the printer’s manuscript to the probable reading of the original manuscript, no longer extant here.