Although unto is archaic for speakers of modern English, its occurrence is quite prevalent in the Book of Mormon text. In fact, for the clause “the voice came”, we have 12 other occurrences in the text with the preposition unto but only 3 with to (each marked below with an asterisk):
Note, in particular, the occurrence of unto nearby in 2 Nephi 31:14 (“thus came the voice of the Son unto me”). Yet variation is possible: Mosiah 24:13 reads to, but the nearby Mosiah 24:16 reads unto. Of course, modern English speakers expect to rather than the archaic unto. Here in the printer’s manuscript for 2 Nephi 31:12, Oliver Cowdery initially wrote to (the expected preposition in modern English); then almost immediately he corrected the manuscript to read unto (the level of ink flow for the crossout and the supralinear unto is unchanged).
Summary: In 2 Nephi 31:12 follow Oliver Cowdery’s virtually immediate correction in 𝓟 that restored the biblically styled preposition unto.