mumble vs utter

mumble is a noun but utter is not a noun.

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

As verbs, utter is a hypernym of mumble; that is, utter is a word with a broader meaning than mumble:

Other hypernyms of mumble include mouth, speak, talk, verbalise, verbalize.

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

mumble (adjective)	utter (adjective)
	complete and absolute
	without qualification; used informally as (often pejorative) intensifiers

mumble (verb)	utter (verb)
talk indistinctly; usually in a low voice	express in speech
grind with the gums; chew without teeth and with great difficulty	articulate; either verbally or with a cry, shout, or noise
	express audibly; utter sounds (not necessarily words)
	put into circulation