mumbler vs speaker

mumbler and speaker both are nouns.

Word	Noun	Adjective	Verb	Adverb
mumbler	Yes	No	No	No
speaker	Yes	No	No	No

As nouns, speaker is a hypernym of mumbler; that is, speaker is a word with a broader meaning than mumbler:

mumbler: a person who speaks softly and indistinctly
speaker: someone who expresses in language; someone who talks (especially someone who delivers a public speech or someone especially garrulous)

Other hypernyms of mumbler include talker, utterer, verbaliser, verbalizer.

mumbler (noun)	speaker (noun)
a person who speaks softly and indistinctly	electro-acoustic transducer that converts electrical signals into sounds loud enough to be heard at a distance
	someone who expresses in language; someone who talks (especially someone who delivers a public speech or someone especially garrulous)