他一边玩手机一边心不在焉地'嗯哼'了一声。

While scrolling on his phone, he absentmindedly said '___mm-hum' to show he was listening.

她一边看电视一边用'嗯哼'应付着对话，几乎没在听。

She kept responding with '___mm-hum' while watching TV, barely listening to the conversation.