holler vs sound off

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

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

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

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

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

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