nag vs sound off

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

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

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

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

nag (noun)	sound off (noun)
an old or over-worked horse
someone (especially a woman) who annoys people by constantly finding fault

nag (verb)	sound off (verb)
remind or urge constantly	express complaints, discontent, displeasure, or unhappiness
bother persistently with trivial complaints	express one's opinion openly and without fear or hesitation
worry persistently	start playing