trail away/off
phrasal verb with trail verb uk/treɪl/ us/treɪl/
（说话声或类似声响）逐渐减弱到停止，逐渐消失
When a person's voice or a similar sound trails away/off, it becomes quieter and less confident and then stops completely.
His voice trailed off as he saw the look on her face. 当他看到她脸上的表情时，他的说话声越来越小，最后听不见了。