There is nothing in the Bible to indicate that Satan is omniscient. There are no verses that say he knows everything or that he can read our thoughts. But he is very adept at predicting human behavior because he’s seen it operate for so long.  He can anticipate what you might do in a given situation without knowing your thoughts because of his knowledge of humankind and because he has a supernatural mind.

But in terms of being omniscient and being able to read your thoughts (as God can), the Bible does not support that idea at all. It never tells us that angels are omniscient. And if a holy angel isn’t omniscient, neither is a fallen one. So, Satan can’t read our thoughts, even if he’s great at predicting human behavior because he’s seen so much of it. So, there is nothing in the Bible that says Satan can read our thoughts. Certainly demons can hear what we say. They can understand what we say., they are very good at predicting the common responses of man.

As far as speaking out loud when we pray.. WHO CARES!! As long as you have the whole armor of God on they can listen all they want to. . The key is to live a righteous, Spirit-filled life and to trust in the sovereign power of God.