。
She spoke with a trembling voice.
Literal
She [topic-は] voice [object-を] trembling spoke.
声を震わせて means 'making her voice tremble' — 震わせる is the causative of 震える (to tremble/shake). The te-form connects it to 話した, describing the manner of speaking: she spoke while her voice trembled. This is a vivid, physical detail that immediately conveys emotional intensity — fear, anger, sadness, or overwhelming feeling.