。
It turned out she was a great musician.
Literal
She [topic-は] great musician is [thing-こと] [subject-が] became-clear.
The frame ~であることがわかった ('it turned out that...') is built from である (literary copula) + こと (nominalizer turning the clause into a fact-noun) + わかる ('become clear, be understood'). Used when a previously unknown identity, status, or quality is revealed. The discovery framing — past わかった — implies the speaker (and possibly the world at large) only learned this later.