sound off vs holler

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

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

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

sound off (noun)	holler (noun)
	a very loud utterance (like the sound of an animal)
	a small valley between mountains

sound off (verb)	holler (verb)
express complaints, discontent, displeasure, or unhappiness	complain
express one's opinion openly and without fear or hesitation	utter a sudden loud cry
start playing	shout out