。
She lowered her voice.
Literal
She [topic-は] voice [object-を] hushed.
声を潜める (こえをひそめる) means 'to lower one's voice' or 'to speak in a hushed tone.' 潜める means 'to hide, to suppress, to hush.' Unlike 声を小さくする (the mechanical 'make voice smaller'), 声を潜める implies deliberate secrecy or discretion — speaking softly so others can't hear.