sound alphabet vs ABC's

sound alphabet and ABC's both are nouns.

Word	Noun	Adjective	Verb	Adverb
sound alphabet	Yes	No	No	No
ABC's	Yes	No	No	No

As nouns, ABC's is a hypernym of sound alphabet; that is, ABC's is a word with a broader meaning than sound alphabet:

sound alphabet: an alphabet of characters intended to represent specific sounds of speech
ABC's: a character set that includes letters and is used to write a language

Other hypernyms of sound alphabet include ABC, ABCs, alphabet.

sound alphabet (noun)	ABC's (noun)
an alphabet of characters intended to represent specific sounds of speech	a character set that includes letters and is used to write a language