By the sound he knew them to be human, and likely two people.

Now he was right on the other side of the door, by the sound.

And by the sound of things outside he didn't want to.

By the sound, there was a great deal more coming.

We have a few minutes yet, by the sound of it.

He could tell by the sound of her voice that she had turned again.

I could believe it, just by the sound of her name.

You could tell by the sound that something was wrong.

With the window down I could have done it by sound alone.

Five or six men, by the sound of it, though he couldn't be sure until they showed themselves.