long vowel vs vowel sound

long vowel and vowel sound both are nouns.

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

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

long vowel: A vowel which is normally pronounced somewhat longer than other vowels (usually around 1½ to double length).
vowel sound: a speech sound made with the vocal tract open

Other hypernyms of long vowel include vowel.

long vowel (noun)	vowel sound (noun)
A vowel which is normally pronounced somewhat longer than other vowels (usually around 1½ to double length).	a speech sound made with the vocal tract open