The original text reads here in the plural (“and would not hearken unto their words”). The 1879 LDS edition introduced the singular word, probably accidentally. The plural words was restored to the LDS text in the 1920 edition by reference to the 1830 edition or some other early edition. The Book of Mormon text consistently uses the plural words whenever the verb hearken refers to the word(s) of more than one person (eight times, including here in Ether 11:13). Besides the example here in Ether 11:13, there are two more examples that refer to hearkening unto the words of the prophets:
The plural words here in Ether 11:13 is undoubtedly correct.
Summary: Maintain the plural words with the verb hearken in Ether 11:13 since this is the earliest reading; moreover, when the text refers to hearkening unto the word(s) of more than one person, we consistently get the plural words.