nasal vowel vs vowel sound

nasal vowel and vowel sound both are nouns.

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

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

nasal vowel: (phonetics) A vowel that is produced with a lowering of the velum so that air escapes both through nose as well as the mouth.
vowel sound: a speech sound made with the vocal tract open

Other hypernyms of nasal vowel include vowel.

nasal vowel (noun)	vowel sound (noun)
(phonetics) A vowel that is produced with a lowering of the velum so that air escapes both through nose as well as the mouth.	a speech sound made with the vocal tract open