duet vs piece of music

duet and piece of music both are nouns.

Word	Noun	Adjective	Verb	Adverb
duet	Yes	No	No	No
piece of music	Yes	No	No	No

As nouns, piece of music is a hypernym of duet; that is, piece of music is a word with a broader meaning than duet:

Other hypernyms of duet include composition, musical composition, opus, piece.

duet (noun)	piece of music (noun)
(ballet) a dance for two people (usually a ballerina and a danseur noble)	a musical work that has been created
a musical composition for two performers
a pair who associate with one another
two performers or singers who perform together
two items of the same kind