Certainly there is power in the spoken word. We tend to be more confident in what we pray when we speak it. For others to agree with us in prayer, they must hear our prayer. However, there is also power in silent prayer.
Is silent prayer biblical?
In the Old Testament, Hannah is seen praying silently (1 Samuel 1:12-13), and her request was granted. Though Jesus does not specifically mention "silent prayer", He does talk about praying alone (Matthew 6:6). First Thessalonians 5:17 tells us to pray without ceasing. Obviously we cannot be constantly speaking aloud to God, so there must be such a thing as a prayerful attitude or silent prayer. Certain situations – such as praying during a meeting, praying for wisdom in a conversation, praying while giving a presentation, or praying for the person to whom you are talking – seem to necessitate silent prayer. We are also told about praying in the Spirit (Ephesians 6:18). Romans 8:26 says that sometimes we do not know what to pray and the Holy Spirit intercedes for us. In many ways, this seems to be a silent prayer.
God can hear our thoughts (Psalm 139:23). When we pray silently, He is still there. Silent prayer is still a prayer of confidence and faith.
Why pray? What is the purpose of prayer?
How does a person pray in Jesus' name?
Persistent prayer - Is it biblical? Is it acceptable to repeatedly pray for the same thing, or should we ask only once?
How can I recognize the voice of God?
Why are Christians encouraged to have daily devotions or quiet times?
Truth about Prayer