mutter vs murmur

mutter and murmur both are nouns.

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

As nouns, mutter and murmur are synonyms defined as:

Other synonyms of mutter include grumble, grumbling, murmuring, muttering.

As nouns, mutter and murmur are synonyms defined as:

mutter and murmur: a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech

Other synonyms of mutter include murmuration, murmuring, mussitation, muttering.

As verbs, mutter and murmur are synonyms defined as:

mutter and murmur: make complaining remarks or noises under one's breath

Other synonyms of mutter include croak, gnarl, grumble.

mutter (noun)	murmur (noun)
a complaint uttered in a low and indistinct tone	a complaint uttered in a low and indistinct tone
a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech	a schwa that is incidental to the pronunciation of a consonant
	a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
	an abnormal sound of the heart; sometimes a sign of abnormal function of the heart valves

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