。
She's concentrating on the music.
Literal
She [topic-は] music [on-に] is-concentrating.
~に集中する ('to concentrate on / to focus on') is the standard pattern, with に marking the focal target. 集中している (the resulting-state form) describes the ongoing condition of being focused, distinct from the snapshot moment of 集中する ('begins to concentrate'). The verb is sahen — 集中 + する — a compact noun-plus-する construction common to Sino-Japanese vocabulary.