mumble vs speak

mumble is a noun but speak is not a noun.

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

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

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

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

mumble (verb)	speak (verb)
talk indistinctly; usually in a low voice	express in speech
grind with the gums; chew without teeth and with great difficulty	exchange thoughts; talk with
	use language
	give a speech to
	make a characteristic or natural sound