complaint vs muttering

complaint and muttering both are nouns.

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

As nouns, muttering is a hyponym of complaint; that is, muttering is a word with a more specific, narrower meaning than complaint:

Other hyponyms of complaint include grievance, grumble, grumbling, murmur, murmuring, mutter, jeremiad, kvetch, pet peeve, whimper, whine.

complaint (noun)	muttering (noun)
an expression of grievance or resentment	a complaint uttered in a low and indistinct tone
(formerly) a loud cry (or repeated cries) of pain or rage or sorrow	a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
(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