。
She likes quiet songs.
Literal
She [topic-は] quiet songs [subject-が] likes.
静かな曲が好き uses the XがYだ pattern with 好き — 曲 (きょく, song/piece of music) is the subject of 好き rather than the object, which is why が appears rather than を. This is a fundamental feature of how 好き works in Japanese: the liked thing takes が, not を.