vowel vs speech sound

vowel and speech sound both are nouns.

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

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

vowel: a speech sound made with the vocal tract open
speech sound: (phonetics) an individual sound unit of speech without concern as to whether or not it is a phoneme of some language

Other hypernyms of vowel include phone, sound.

vowel (noun)	speech sound (noun)
a speech sound made with the vocal tract open	(phonetics) an individual sound unit of speech without concern as to whether or not it is a phoneme of some language
a letter of the alphabet standing for a spoken vowel