God speaks in his word and he also speaks elsewhere, like in nature and in our consciences. What's the difference?

The difference is authority and inerrancy.

In the Scriptures, God speaks with absolute authority and without any error. That's the way Jesus thought about the Old Testament. It doesn't err, it doesn't make any mistakes. You follow it, and you don't make any mistakes when God speaks that way.

Now, that might sound like, "Oh, you think God makes mistakes in nature?" or "Oh, you think God makes mistakes in consciences?"

No, that's not what I'm saying. I'm saying we have in this Book an objective thing we call the Word of God. It makes no mistakes.

Now in that sense, God's voice in nature is flawless. But, hearing that voice or construing a meaning there, we easily make mistakes.

And even as I'm talking I'm realizing that I'm going to have to make some more distinctions, aren't I? Because God's word is true wherever he speaks. If it's in a conscience, God is true. If it's in nature, God is true. If it's in the Bible, God is true.

But here we have the words and we discern them with our mind. We can say, "That's what it says."

It's not easy with a cloud in the sky, like, "That's what God is saying." Well, what? Or, "My conscience is troubled by (whatever), so I know now what God is saying. It's passing through my mind." And "my mind" is not necessarily infallible like the Bible is infallible.