Since the greatness of prayer nourishes both the upper and lower worlds through it, as it is a pillar of the universe, and even to the upper worlds abundance will flow through it, therefore the Holy Zohar cautioned to pray quietly until even his own ears do not hear his voice. The reason for this is so that outsiders will not become jealous of it because of the greatness of its virtues. As our teacher and master Rabbi Chaim Vital of blessed memory wrote in "Olat Tamid," page 56b, and this is his language: "Prayer does not enter any firmament unless it is a prayer that is not audible to others. Therefore, in the reading of Shema, they ordained that one should hear it with his ears. But here, if the prayer is audible to his ears, it is impossible, for outsiders hear it and interfere with it, and when it is whispered and is not audible to his ears, then it is very spiritual, of the nature of the spirit of man, which is internal and more spiritual than the external inclination, and therefore it is not seen. This contradicts the opinion of the Maharik, in the Beit Yosef, who ruled that it is better to hear it with one's ears. And see in the Baraita and in the Jerusalem Talmud and you will find that his opinion is the opposite, and similarly, one who reads the Torah scroll, whether he reads it aloud or whispers, one horn of faith is missing, for prayer and the reading of the Torah are both from the highest concealment, and for this reason we recite them in a whisper and so on." See there for his exact words.