Q: Should Christians pray out loud when praying by ourselves? I know God knows our thoughts but is there something more powerful about the spoken word during prayer?
You're asking if a spoken prayer is more powerful than a prayer expressed without words from the heart. I would have to say no, for two reasons.
1. There is nothing in the Word that directs us specifically to pray out loud as a means of making our prayers more effective.
2. There are biblical examples of prayers being offered up without words that were answered by the Lord.
Example one: Hannah, (as recorded in 1 Samuel 1) was a woman who poured out her heart to God in prayer, and we are told that as she prayed "...only her lips moved, and her voice was not heard." (:13) And yet, God powerfully answered her petition.
Example two: Nehemiah (in Nehemiah 2) was tending to King Artaxerxes in his official role as Cupbearer, and during a conversation with the King, Nehemiah relates that he "prayed to the God of heaven" before answering a specific question that had been put to him by the king. Nehemiah hardly had time to go off and pray on his own, so it seems obvious from the text that he shot up a quick and silent prayer to the Lord his God, which was wonderfully answered.
Some believe the best passage in support of silent prayer is 1 Thessalonians 5:17 where the Apostle Paul exhorts us to “Pray without ceasing.” This verse speaks of a constant and ongoing communication with God throughout our day, which would most certainly involve verbal and non-verbal petitions and thanksgiving.
We're clearly told in the Word that our heavenly Father knows what we need before we ask Him (Matthew 6:8). And yet He wants us to come to Him and to pour out our needs before His throne of grace. There is certainly a place for verbal prayers, and whenever possible I try to pray out loud. In my own case, it just gives me a greater sense of talking to my Lord. But obviously that's not always possible. When voicing my prayers out loud isn't feasible, I move into silent prayer confident that my Father in Heaven hears with equal clarity and eagerness.