。
She's interested in music.
Literal
She [topic-は] music [in-に] interest [subject-が] exists.
~に興味がある is the static framing — 'interest exists in X,' the natural Japanese way to describe an ongoing curiosity or affinity. Its close cousin ~に興味を持つ ('hold/take interest in') is a touch more active, painting interest as something one acquires; the bare ある version paints interest as something one simply has. が rather than を marks 興味, since with ある (which is intransitive) the thing existing must be the subject.