Here the earliest textual sources have the plural words. The 1906 LDS edition substituted the singular word for the plural. Subsequent LDS editions have retained the correct plural since the 1906 edition never served as a copytext. Overall, there are 42 instances in the text of “hearken (un)to the words of X”, but only 5 of the singular “hearken (un)to the word of X”; thus the plural dominates. Here in Helaman 12:4, we have the only passage that refers to the word(s) of Satan. But there are references to hearkening (or not hearkening) to the words of wicked people, as in Jacob 7:23: “and they searched the scriptures and hearkened no more to the words of this wicked man”. For each case of “hearken (un)to the word(s) of X”, the critical text will follow the grammatical number of the earliest textual source(s), thus “the words of the evil one” here in Helaman 12:4.
Summary: Maintain in Helaman 12:4 the plural words in “hearken unto the words of the evil one”, the reading of the earliest extant source (here the printer’s manuscript).