Oliver Cowdery initially wrote “the voice of the good shepherd”, a highly expected phrase deriving from the biblical language in John 10:11–16, especially “I am the good shepherd” and “they shall hear my voice and there shall be one fold and one shepherd”. But almost immediately after writing down this expected phrase in 𝓟, Oliver crossed out good shepherd and supralinearly inserted the word Lord (the level of ink flow is unchanged).
The Book of Mormon has other examples of “the voice of the good shepherd” (4 occurrences) and “the voice of the Lord” (26 occurrences). There are three occurrences of “the voice of the good shepherd” in Alma 5 (verses 38, 41, and 57) and one in Helaman 7:18, so Oliver Cowdery’s initial error in Mosiah 16:2 may have been based more on his familiarity with the biblical text than on him remembering what he had written down in 𝓞 for Alma 5 and Helaman 7 about half a year earlier. In either case, there would have been no reason why Oliver should have made this change in 𝓟 for Mosiah 16:2 except that 𝓞 read that way. The critical text will maintain the corrected reading in 𝓟.
Summary: Retain in Mosiah 16:2 the corrected reading in 𝓟: “the voice of the Lord”; Oliver Cowdery’s correction undoubtedly agrees with the reading of the original manuscript, no longer extant here.