diction vs mumbling

diction and mumbling both are nouns.

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

As nouns, mumbling is a hyponym of diction; that is, mumbling is a word with a more specific, narrower meaning than diction:

diction: the articulation of speech regarded from the point of view of its intelligibility to the audience
mumbling: indistinct enunciation

diction (noun)	mumbling (noun)
the manner in which something is expressed in words	ineffectual chewing (as if without teeth)
the articulation of speech regarded from the point of view of its intelligibility to the audience	indistinct enunciation