。
I was surprised to hear her voice in the next room.
Literal
Next [of-の] room [in-に] her [of-の] voice [object-を] heard surprised.
~て驚く is a result-producing te-form: '(the thing that happened) led to surprise.' Here the hearing leads to the surprise. Note 彼女の声を聞いて — the active 聞く ('to listen to, hear deliberately') takes を on what was heard, distinct from the spontaneous-perception 聞こえる ('to be audible'), which takes が.