Stan Larson has suggested that in this passage the words had been may have been accidentally lost (see footnote 19 on page 568 of his article “Conjectural Emendation and the Text of the Book of Mormon”, Brigham Young University Studies 18/4 : 563–569). Such an emendation would increase the parallelism with the preceding “there had been false Christs and their mouths had been shut”.
Other examples in the text involving had been show that in conjoined verb phrases, ellipsis of the helping verbs occurs most of the time:
There is one example showing a conjoining of had been with was (or were since the 1837 edition):
This example suggests an alternative emendation for the Words of Mormon 1:15: “and their mouths had been shut and they were punished according to their crimes”.
One important difference, however, in the Words of Mormon 1:15 is that the conjunctive element is a conjoined clause and not a predicate; specifically, there is a subject they in the conjoined clause, unlike all the other examples listed above. This kind of ellipsis is referred to as gapping, and one particular type of gapping involves the omission of helping verbs. Gapping is discussed, for instance, on pages 974–975 in Randolph Quirk, Sidney Greenbaum, Geoffrey Leech, and Jan Svartvik: A Comprehensive Grammar of the English Language (London: Longman, 1985). They provide the following example (in note a) of what is referred to as a rather rare kind of gapping: “the fortress had been attacked and its commander killed”. This example precisely patterns the reading in the Words of Mormon 1:15, “their mouths had been shut and they punished.” Since gapping is grammatical in English, the critical text will allow it in the Book of Mormon, despite its rarity. Of course, the possibility remains that the gapping here in the Words of Mormon 1:15 is actually due to an error in the early transmission of the text.
Summary: Maintain the occurrence of gapping in the Words of Mormon 1:15: “and their mouths had been shut and they punished according to their crimes”.