mouth vs sound out

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

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

mouth (noun)	sound out (noun)
the opening of a jar or bottle
the externally visible part of the oral cavity on the face and the system of organs surrounding the opening
the opening through which food is taken in and vocalizations emerge
an impudent or insolent rejoinder
the point where a stream issues into a larger body of water
an opening that resembles a mouth (as of a cave or a gorge)
a person conceived as a consumer of food
a spokesperson (as a lawyer)

mouth (verb)	sound out (verb)
express in speech	try to learn someone's opinions and intentions
articulate silently; form words with the lips only	speak, pronounce, or utter in a certain way
touch with the mouth