。
She turned around when she heard his voice.
Literal
She [topic-は] his [genitive-の] voice [object-を] heard-and turned-around.
The te-form 聞いて links cause/sequence ('heard and so'). 聞く ('hear, listen') takes its object directly with を — distinct from the intransitive 聞こえる ('be audible'), which takes が. 振り向く is a 複合動詞 combining 振る ('shake, swing') + 向く ('face') — 'turn (one's body/head) around.' Used specifically for turning to look at something or someone behind you, in response to a call or noise.