Isaiah 52:8 (King James Bible) thy watchmen shall lift up the voice with the voice together shall they sing
As explained under Mosiah 15:29, the 1953 RLDS edition tended to replace the singular voice with the plural voices. The earliest textual sources here in 3 Nephi 20:32 and the corresponding King James passage in Isaiah 52:8 have the singular voice, as will the critical text of the Book of Mormon. Also note that here the Book of Mormon version reads “lift up their voice”, unlike the King James version (“lift up the voice”). For discussion of this difference, see under Mosiah 15:29.
There is also one other difference here in 3 Nephi 20:32, namely, an extra and before the second clause. In comparison, Isaiah 52:8 and the three other Book of Mormon quotations of this passage (in Mosiah 12:22, Mosiah 15:29, and 3 Nephi 16:18) do not have this extra and: “thy watchmen shall lift up the(ir) voice / with the voice together shall they sing”. Even so, the and here in 3 Nephi 20:32 will be maintained since both 𝓟 and the 1830 edition have it, which implies that 𝓞 did too. And as noted for other biblical passages quoted more than once in the Book of Mormon, the direct quotations may nonetheless vary slightly. See, for instance, the discussion regarding face(s) under 2 Nephi 6:7 and 2 Nephi 26:20. For each of these cases of variation in biblical quotes, we follow the earliest Book of Mormon reading, thus maintaining the extra and here in 3 Nephi 20:32 (but allowing for the lack of it in the three other quotations of the same Isaiah passage).
Summary: Maintain in 3 Nephi 20:32 the earliest reading, the singular voice and the conjunction and that precedes the second clause (“and with the voice together shall they sing”); in the biblical quotations, we follow the earliest Book of Mormon reading unless there is clear evidence of some error in transmission.