mumbling vs chewing

mumbling and chewing both are nouns.

Word	Noun	Adjective	Verb	Adverb
mumbling	Yes	No	No	No
chewing	Yes	No	No	No

As nouns, chewing is a hypernym of mumbling; that is, chewing is a word with a broader meaning than mumbling:

mumbling: ineffectual chewing (as if without teeth)
chewing: biting and grinding food in your mouth so it becomes soft enough to swallow

Other hypernyms of mumbling include chew, manduction, mastication.

mumbling (noun)	chewing (noun)
ineffectual chewing (as if without teeth)	biting and grinding food in your mouth so it becomes soft enough to swallow
indistinct enunciation