mutter vs speak

mutter is a noun but speak is not a noun.

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

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

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

mutter (noun)	speak (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)	speak (verb)
make complaining remarks or noises under one's breath	express in speech
talk indistinctly; usually in a low voice	exchange thoughts; talk with
	use language
	give a speech to
	make a characteristic or natural sound