It really depends. For me, I feel the spiritual battle and see it's evidence readily, and I believe that satan does not have the same attributes as God, he can't know our thoughts and what is ahead of us, he can only play off of what we do and say and use that to tempt us. So for me, praying out loud is a blatant attack on the enemy. Praying in my heart or writing my prayers down rather than out loud is something really close between me and God, and it's more like having a conversation with Him and seeking Him with answers. Like when I'm praying with someone else then I also tend to view it as an attack on the Enemy because usually that's what it is, pleading God to pierce the Enemy's attacks and keep us on the right path, on His plan for our lives.