。
She was overheard playing the violin.
Literal
She [topic-は] violin [object-を] play [nominalizer-の] [object-を] was-heard.
A passive built around a nominalised clause: バイオリンを弾く ('play the violin') is wrapped with の to make it a noun phrase, then made the object of 聞かれた (passive of 聞く, 'to hear / be heard'). The result is a particular Japanese flavour of passive sometimes called the 'indirect' or 'suffering' passive — the subject (彼女) bears the consequences of someone else's perception, with the unspoken implication that being heard wasn't what she intended. Note the double を: one for 弾く's object (the violin), another for 聞く's object (the act of playing). Multiple を on the surface is permitted here because they belong to different nested clauses.