I could hear him mumbling to himself.

我听到他在喃喃自语。

What are you mumbling to yourself?

你在嘀咕什么？

He closed his eyes, fell to mumbling, and presently was silent.

他闭上眼睛，喃喃自语起来，不一会儿就沉默了。

Do you think that people are always mumbling?

你是否觉得人们总是含糊不清地说话？

Stop mumbling! I can't understand what you say.

不要讲话隐约不清的，我听不懂你在说什么。