murmur vs mutter

murmur and mutter both are nouns.

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

As nouns, murmur and mutter are synonyms defined as:

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

As nouns, murmur and mutter are synonyms defined as:

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

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

As verbs, murmur and mutter are synonyms defined as:

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

Other synonyms of murmur include croak, gnarl, grumble.

murmur (noun)	mutter (noun)
a complaint uttered in a low and indistinct tone	a complaint uttered in a low and indistinct tone
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
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

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