We know that G-D is all knowing, and all powerful. Therefore, when we pray, why does it have to be out loud? There are some brachos that you are not even יוצא if you say it quietly! Then why do we need to say them out loud if G-D knows exactly what we need and what is good for us, even more than we do?
Praying out loud is derived in the Talmud from the prayer of Chanah.
אמר רב המנונא כמה הלכתא גברוותא איכא למשמע מהני קראי דחנה וחנה היא מדברת על לבה מכאן למתפלל צריך שיכוין לבו רק שפתיה נעות מכאן למתפלל שיחתוך בשפתיו וקולה לא ישמע מכאן שאסור להגביה קולו בתפלתו
R. Hamnuna said: How many most important laws can be learnt from these verses relating to Hannah! Now Hannah, she spoke in her heart: from this we learn that one who prays must direct his heart. Only her lips moved: from this we learn that he who prays must frame the words distinctly with his lips. But her voice could not be heard: from this, it is forbidden to raise one's voice in the Tefillah.
In the very same passage where we are told that you must enunciate the words, we are in fact also exhorted to not say them loudly. The Talmud earlier (24b) provides a reason for this:
המשמיע קולו בתפלתו הרי זה מקטני אמנה המגביה קולו בתפלתו הרי זה מנביאי השקר
'One who says the Tefillah so that it can be heard is of the small of faith; he who raises his voice in praying is of the false prophets;
Rashi there explains:
כאילו אין הקב"ה שומע תפלת לחש ומגביה הרבה
As if God does not hear quiet prayer, and he raises [his voice] much.
We see here that the halacha is aware of the theological implications of specific ways of praying, and seeks to avoid them. That there is then a requirement to speak the words at all points to there being some benefit in doing so.
R. Jacob Ben Asher (Tur O.C. 101) says the following:
והדעת נותנת שיותר טוב להשמיע לאזניו כי אז יוכל לכוין יותר
And logic dictates that it is better to make it audible to his ears, because then he will be able to concentrate more.
This reason is repeated by R. Mordechai Yaffe (Levush O.C. 101:2) as well:
אבל מ"מ צריך להשמיע לאזניו שאז יוכל לכון יותר
But nevertheless he must make it audible to his ears because then he will be able to concentrate better.
R. Abraham Gombiner (Magen Avraham O.C. 101:2) offers what appears to be a mystical rationale:
כי הקול מעורר למעלה כידוע
Because the sound stirs up in the Upper Realms, as is known.
Note that this is all assuming that there is actually a requirement for some level of audibleness. However, this itself is in fact a matter of debate based on conflicting passages.
Since the Almighty is All-Knowing, why do we need to pray at all? He knows what's best for us. Based on Rav Hirsch's commentary on the word פלל - prayer/judging, we look at ourselves to see how we can develop our relationship to be more in sync with our Divine Mission. Perhaps praying out loud - enough that you yourself can hear it - will have a stronger effect on the purpose of prayer of waking us up enough to consider whether we're meeting our goals.
I think that G-d knows everything. Anything we or any cleric says G-d already knows. Torah readings what is called davening (praying) the Torah is important. In Hebrew prayer is lehitpaleil, and root of lehitpaleil is p-l-l, which means “judge.” So the meaning of lehitpaleil is “to judge one’s self.” People do not need to say prayer verbally. They can do it while thinking, but saying it verbally helps them to remember their goals to improve.
Commentators differ on G-d's knowledge. Some traditionalists and Spinoza felt that G-d knows everything. People do not have to pray aloud; they can pray while thinking. There is nothing that a human can tell G-d that G-d does not know. But there are wise Jews that disagree. Ralbag, ibn Ezra, and possibly Maimonides felt that G-d knew all that would occur and provided everything with the laws of nature. Thus, G-d does not know all the details. For example, G-d knows about the laws of nature that G-d created; but not people as individuals. In other words, G-d knows things generally but not particularly. Thus, the notion that G-d is all knowing may not be true.