mumbling vs diction

mumbling and diction both are nouns.

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

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

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

Other hypernyms of mumbling include enunciation.

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