complaint vs mutter

complaint and mutter both are nouns.

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

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

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

complaint (noun)	mutter (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

complaint (verb)	mutter (verb)
	make complaining remarks or noises under one's breath
	talk indistinctly; usually in a low voice