croak vs sound off

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

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

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

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

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

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