sound alphabet vs ABCs

sound alphabet and ABCs both are nouns.

Word	Noun	Adjective	Verb	Adverb
sound alphabet	Yes	No	No	No
ABCs	Yes	No	No	No

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

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

Other hypernyms of sound alphabet include ABC's, ABC, alphabet.

sound alphabet (noun)	ABCs (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