mumble

含糊地说、咕哝、喃喃而语

He mumbled an apology and hurried out of the room.

他含糊地道了个歉，就匆匆走出房间。

低声自语、喃喃自语

She kept mumbling to herself while looking for her keys.

她一边找钥匙，一边不停地低声自语。

含糊的话、咕哝声

His answer was just a mumble that nobody could understand.

他的回答只是一句谁都听不懂的含糊话语。