Usually I pray silently, even in private. After all, I've always figured God knows what goes on in my head, and I still believe that.
Today, for some unaccountable reason, I had the house to myself and I prayed aloud, in a normal voice, not a whisper, and my prayer time seemed so much more alive. I felt more as if I was talking to God and less as if I was talking to myself. It was also broader and more specific. I prayed about things I'd normally forget and I prayed in greater depth.
And now I find myself wondering why.
Maybe it has to do with the way God built us. Perhaps when we speak with others the part of our brain that engages with our mouths comes into play, and that part of our brain is necessary for us to sense we are really communicating, and since it is made for communication, the communication is more natural and deeper. I don't know; that's pure speculation.
However, I do know that I had a mighty good time, and felt I'd connected to God in a fresh, invigorating way, and I'd like to recommend you give it a try.