muttering vs complaint

muttering and complaint both are nouns.

Word	Noun	Adjective	Verb	Adverb
muttering	Yes	No	No	No
complaint	Yes	No	No	No

As nouns, complaint is a hypernym of muttering; that is, complaint is a word with a broader meaning than muttering:

muttering (noun)	complaint (noun)
a complaint uttered in a low and indistinct tone	an expression of grievance or resentment
a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech	(formerly) a loud cry (or repeated cries) of pain or rage or sorrow
	(civil law) the first pleading of the plaintiff setting out the facts on which the claim for relief is based
	(criminal law) a pleading describing some wrong or offense
	an often persistent bodily disorder or disease; a cause for complaining