If you talk out loud to the Lord, are you worried the devil will know your plans and thwart them?

This verse means that the power and strength of God residing within believers (the Holy Spirit) is greater than the power of the evil one (Satan) and his influence in the world, giving Christians confidence and the ability to overcome challenges.
The belief that Satan will target our weaknesses, only emphasizes the importance of seeking strength from God and resisting temptation through prayer, scripture, and reliance on faith. Jesus' own experiences in the wilderness, where he was tempted by the devil, are seen as a demonstration of the power of temptation and the importance of resisting it!
This verse is found in 1 John 4:4, within a passage that warns against false teachers and emphasizes the importance of discerning truth from error:
"You are of God, little children, and have overcome them, because greater is He who is in you than he who is in the world".
It confirms that believers, who have the Holy Spirit dwelling within them, are more powerful and capable than those who are under the influence of the world's evil forces.
The "he who is in you" refers to the Holy Spirit, while "he who is in the world" refers to the evil one (Satan). Even if the devil attempts to interfere, God's power and protection are ultimately greater.
Instead of worrying about the devil's potential knowledge, focus on your faith and trust in God's guidance and protection. God is ultimately more powerful than any evil influence. Satan is not omniscient... knowing everything, as God does. He cannot hear your prayers when you pray silently, but regardless whether you pray in your head or out loud, you belong to the living God and no matter what any spirit or man has to throw at you, you have faith and strength to overcome it through His power.

"Put on the full armor of God so that you can fight against the devil’s evil tricks. Our fight is not against people on earth but against the rulers and authorities and the powers of this world’s darkness, against the spiritual powers of evil in the heavenly world. That is why you need to put on God’s full armor. Then on the day of evil, you will be able to stand strong!! And when you have finished the whole fight, you will still be standing. So stand strong, with the belt of truth tied around your waist and the protection of God, as you live according to His Word. On your feet, wear the Good News of peace to help you stand strong. Also use the shield of faith with which you can stop all the burning arrows of the Evil One. Accept God’s salvation as your helmet, and take the sword of the Spirit, which is the Word of God."
The Bible has a lot to say about fear, in fact it says “do not be afraid” some 365 times... one time for every day of the year. When you feel afraid or persecuted, do not be afraid, God is with you always.
Prayer is a time when you’re in communion with God. It’s when you should be in the Spirit, being led by the Holy Spirit. He casts out all fear. You speak with your Heavenly Father without fear, because of your faith in Jesus and your position in Him as an adopted child of God the Father.
If I wanted to spend time with a loved one and someone else didn’t want me to, I wouldn’t give in to what they want. I would do it anyway. If they wanted to waste their time trying to stop me, that’s on them. I’m not letting it affect my relationship. Same when speaking with God.
Let yourself be led by the Spirit more. Let Him teach you how to block out distractions. Let Him teach you how to cast out fear, build up your knowledge and your faith, your trust, your confidence, and your boldness!
Jesus said, there is no fear in perfect love, and there's no more perfect love, than God's. 1 John 4:18