sound off vs croak

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

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

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

sound off (noun)	croak (noun)
	a harsh hoarse utterance (as of a frog)

sound off (verb)	croak (verb)
express complaints, discontent, displeasure, or unhappiness	pass from physical life and lose all bodily attributes and functions necessary to sustain life
express one's opinion openly and without fear or hesitation	make complaining remarks or noises under one's breath
start playing	utter a hoarse sound, like a raven