Here the 1892 RLDS edition accidentally replaced the preposition to with unto, perhaps because of the following unto (“and give heed unto it”) or because “hearken unto” is much more frequent in the Book of Mormon text than “hearken to” (81 versus 12 occurrences). Nonetheless, “hearken to” does occur, so there is no reason to think that the use of “hearken to” in 𝓞 for 1 Nephi 16:3 is an error.
The tendency to replace “hearken to” with “hearken unto” is also found in the Book of Mormon quote from Isaiah 51:1:
This 1830 change was probably made, whether intentional or not, because “hearken unto” is more frequent and biblical sounding. For instance, the preposition unto is found in the following verses 4 and 7, which both have “hearken unto me”. Interestingly, Isaiah 51 in the King James Bible shows the same variation as the original text of the Book of Mormon—namely, “hearken to me” in verse 1 but “hearken unto me” in verses 4 and 7. For further discussion, see 2 Nephi 8:1.
Summary: Maintain wherever it occurs the original preposition to in “hearken to”; although less frequent than unto with the verb hearken, the preposition to is still possible.