diphthong vs vowel sound

diphthong and vowel sound both are nouns.

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

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

diphthong: a vowel sound that starts near the articulatory position for one vowel and moves toward the position for another
vowel sound: a speech sound made with the vocal tract open

Other hypernyms of diphthong include vowel.

diphthong (noun)	vowel sound (noun)
a vowel sound that starts near the articulatory position for one vowel and moves toward the position for another	a speech sound made with the vocal tract open