sound off vs mutter

sound off is not a noun while mutter is a noun.

Word	Noun	Adjective	Verb	Adverb
sound off	No	No	Yes	No
mutter	Yes	No	Yes	No

As verbs, mutter is a hyponym of sound off; that is, mutter is a word with a more specific, narrower meaning than sound off:

sound off (noun)	mutter (noun)
	a complaint uttered in a low and indistinct tone
	a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech

sound off (verb)	mutter (verb)
express complaints, discontent, displeasure, or unhappiness	make complaining remarks or noises under one's breath
express one's opinion openly and without fear or hesitation	talk indistinctly; usually in a low voice
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