In fact, the Bible contains many examples of God’s servants who prayed out loud, and there is no hint that these ones feared that the Devil would hear them.
بِشَكْلٍ مُمَاثِلٍ ،‏ لَا يَجِبُ أَنْ نَخَافَ مِنَ ٱلصَّلَاةِ بِصَوْتٍ عَالٍ لِأَنَّهُ يَسْمَعُنَا .‏