low vowel vs vowel sound

low vowel and vowel sound both are nouns.

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

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

low vowel: a vowel that you pronounce with your tongue on the bottom of your mouth.
vowel sound: a speech sound made with the vocal tract open

Other hypernyms of low vowel include vowel.

low vowel (noun)	vowel sound (noun)
a vowel that you pronounce with your tongue on the bottom of your mouth.	a speech sound made with the vocal tract open