I couldn't hear clearly what he was mumbling.

我听不清他在咕噜地说什么。

The girl with braces mumbled a piece of chicken.

带牙套的女孩费力嚼一块鸡肉。

I often say some mumble in my sleep.

我经常在睡梦中喃喃自语。

She mumbled something about being too busy.

她嘟囔着说太忙了

But your Mandarin is so good. Just mumble a bit and you'll be fine!

但是你的普通话这么好。可以稍微含糊地唱，你就会表现好的！

He spoke in a low mumble, as if to himself.

他自言自语般地嘟哝着。

He could hear the low mumble of Navarro's voice.

他能听到纳瓦罗的低声咕哝。

"Well," he conceded, "I do sometimes mumble a bit."

“嗯，”他承认道，“我有时说话确实有点儿含糊。”

When they mumble they're saying the Lord's Prayer backards.

他们念咒的时候，是把圣经的祷文倒过来念的。

They mumble, break all the rules of grammar, slur words and so on.

他们咕哝、打破所有语法规则、吞音等等。