semivowel vs speech sound

semivowel and speech sound both are nouns.

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

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

semivowel: a vowellike sound that serves as a consonant
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 semivowel include phone, sound.

semivowel (noun)	speech sound (noun)
a vowellike sound that serves as a consonant	(phonetics) an individual sound unit of speech without concern as to whether or not it is a phoneme of some language