。
Her voice was trembling with anger.
Literal
She [topic-は] anger [with-に] voice [object-を] was-trembling.
怒りに声を震わせていた uses the causative 震わせる (from 震える, 'to tremble') — literally 'was making her voice tremble with anger.' Like 足を滑らせる, this causative doesn't imply intentional action but rather describes an involuntary physical response. The anger causes the trembling; she doesn't choose to tremble. This involuntary-causative pattern is productive in Japanese for body-related expressions.