This quote comes from my Oxford dictionary. I am not sure how I should understand it. It seemingly suggests that verbs following mutterings should take singular form? I've googled it and most of the hits on the first page shows that it should be treated as a plural noun.Muttering N (Uncountable) (also mutterings[pl])
So, I guess the conclusion should be that we use singular form after muttering and plural form after mutterings.
Am I ringht in thinking so?