mumble vs mutter

mumble and mutter both are nouns.

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

As verbs, mumble and mutter are synonyms defined as:

Other synonyms of mumble include maunder, mussitate.

mumble (noun)	mutter (noun)
a soft indistinct utterance	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

mumble (verb)	mutter (verb)
talk indistinctly; usually in a low voice	make complaining remarks or noises under one's breath
grind with the gums; chew without teeth and with great difficulty	talk indistinctly; usually in a low voice