mutter vs verbalize

mutter is a noun but verbalize is not a noun.

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

As verbs, verbalize is a hypernym of mutter; that is, verbalize is a word with a broader meaning than mutter:

Other hypernyms of mutter include mouth, speak, talk, utter, verbalise.

mutter (noun)	verbalize (noun)
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

mutter (verb)	verbalize (verb)
make complaining remarks or noises under one's breath	convert into a verb
talk indistinctly; usually in a low voice	express in speech
	be verbose
	articulate; either verbally or with a cry, shout, or noise