protest vs sound off

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

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

As verbs, sound off is a hypernym of protest; that is, sound off is a word with a broader meaning than protest:

Other hypernyms of protest include complain, kick, kvetch, plain, quetch.

protest (noun)	sound off (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

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