Learn Italian: La sua musica ha un che di poetico e malinconico. - His music has a certain poetic and melancholic something.

The phrase un che di translates roughly to "a certain something" or "a touch of." While che is normally a conjunction or relative pronoun ("that" or "who"), here it is turned into a noun by the indefinite article un. It is used to express a vague, indefinable quality and is typically followed by di + an adjective.

Why are the adjectives poetico and malinconico masculine singular, even though the subject is the feminine la sua musica?

The adjectives are not modifying musica directly; they are modifying the phrase un che (a something). Because un che acts as a masculine singular noun, any adjective that follows di in this structure must also be masculine singular, regardless of the gender of the sentence's main subject.

What is the difference in nuance between using un che di poetico and qualcosa di poetico?

Both structures are grammatically similar and both require the preposition di before an adjective. However, un che di carries a more elusive, literary feeling—like saying the music has "a certain poetic je ne sais quoi." Qualcosa di poetico is more standard and literal, simply meaning it has "something poetic" about it.

The translation says "His music," but couldn't la sua musica also mean "Her music"?