Here we have the singular word in the earliest textual sources, but one wonders if the correct reading might not be the plural words. In fact, the 1906 LDS edition substituted the plural here, although that change may have been unintended. Subsequent LDS editions have followed the earlier reading, word, since the 1906 edition was never used as a copytext.
There are two other occurrences in the text that refer to God’s word(s) hissing forth, and both of these read in the plural:
The first example shows a tendency on Oliver Cowdery’s part to momentarily write the singular word in place of the correct words. Perhaps the same error occurred here in Moroni 10:28 as Oliver (the presumed scribe in 𝓞 for this part of the text) wrote down Joseph Smith’s dictation or later as he copied the text from 𝓞 into 𝓟. David Calabro notes (personal communication) that it would have been especially difficult to hear the difference between word shall and words shall when Joseph dictated the text.
More generally, however, either word or words can be used to refer to God’s word(s). Although there are quite a few examples in the Book of Mormon of these things and other plurals referring to the coming forth of recorded events, there is some minor evidence for using the singular in referring to the word of God, as in the following clear example:
This example argues that the singular word is definitely possible in Moroni 10:28.
A summarizing discussion of the competition between word and words when referring to God’s word(s) can be found under Helaman 6:36. For each case of word(s), the critical text will follow the earliest reading, thus word here in Moroni 10:28.
Summary: Maintain the singular word in Moroni 10:28 since such a reading is possible; the plural reading, words, is also possible, but in this case we follow the reading of the earliest text (thus “and his word shall hiss forth from generation to generation”).