sound off vs protest

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

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

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

sound off (noun)	protest (noun)
	the act of protesting; a public (often organized) manifestation of dissent
	a formal and solemn declaration of objection
	the act of making a strong public expression of disagreement and disapproval

sound off (verb)	protest (verb)
express complaints, discontent, displeasure, or unhappiness	utter words of protest
express one's opinion openly and without fear or hesitation	affirm or avow formally or solemnly
start playing	express opposition through action or words