rounded vowel vs vowel sound

rounded vowel and vowel sound both are nouns.

Word	Noun	Adjective	Verb	Adverb
rounded vowel	Yes	No	No	No
vowel sound	Yes	No	No	No

As nouns, vowel sound is a hypernym of rounded vowel; that is, vowel sound is a word with a broader meaning than rounded vowel:

rounded vowel: (linguistics) A vowel that is pronounced with the lips drawn together and forming a circular opening.
vowel sound: a speech sound made with the vocal tract open

Other hypernyms of rounded vowel include vowel.

rounded vowel (noun)	vowel sound (noun)
(linguistics) A vowel that is pronounced with the lips drawn together and forming a circular opening.	a speech sound made with the vocal tract open