Here is an excerpt that brings before us a different way to think through how we pray. To pray out loud, when alone, may seem like a bit of a strange concept at first, but I would challenge you to give it a try. I have personally found this to be very beneficial in my own prayer life as it keeps me more focused and concentrated in my prayers.
Much of the discussion about [prayer] has taken place within the context of a Western culture in which the majority of people who, when praying privately, do so silently, without either speaking aloud or “mouthing words.” It remains doubtful whether this was often the case in the ancient world.
Praying was very much like reading, where even in private one read “aloud” – to oneself, to be sure, but by “mouthing words” nonetheless – just as children still do until they are “taught” otherwise. The casual evidence for this in the NT, of course, is Philip’s “hearing” the Ethiopian eunuch as he is “reading” from Isaiah (Acts 8:30). So also with praying. Not only does the narrative in Daniel 6 assume that Daniel prayed “aloud” when praying alone, but so does the parable of Jesus in Luke 18:9-14, as does Luke’s narrative about Jesus’ praying in 11:1 as well as the Synoptic narrative about his prayer in the Garden.
In all of these cases the narrator assumes a culture where people prayed “aloud,” that is, articulated for themselves as they prayed.
–from God’s Empowering Presence by Gordon Fee