lisp vs sound out

lisp is a noun but sound out is not a noun.

Word	Noun	Adjective	Verb	Adverb
lisp	Yes	No	Yes	No
sound out	No	No	Yes	No

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

Other hypernyms of lisp include articulate, enounce, enunciate, pronounce, say.

lisp (noun)	sound out (noun)
a speech defect that involves pronouncing `s' like voiceless `th' and `z' like voiced `th'

lisp (verb)	sound out (verb)
speak with a lisp	try to learn someone's opinions and intentions
	speak, pronounce, or utter in a certain way