דתניא לא יברך אדם ברכת המזון בלבו ואם בירך יצא
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דתניא א"ר יהודה משום ר' אלעזר בן עזריה הקורא את שמע צריך שישמיע לאזנו שנאמר שמע ישראל אמר לו ר' מאיר הרי הוא אומר אשר אנכי מצוך היום על לבבך אחר כונת הלב הן הן הדברים
There is a teaching : A man must not say Grace after meals in his heart ; but if he has done so, he has complied with the requirements of the law.
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For there is a teaching : R. Judah says in the name of R. Eleazar b. 'Azariah : He who reads the Shema' must make it audible to his ear ; as it is said, "Hear O Israel." R. Meir said to him : Behold the Scriptures state, "And these words which I command thee this day shall be upon thy heart," i.e. the words depend upon the intention of the heart.
מיתיבי המשמיע קולו בתפלתו הרי זה מקטני אמנה המגביה קולו בתפלתו הרי זה מנביאי השקר מגהק ומפהק הרי זה מגסי הרוח המתעטש בתפלתו סימן רע לו ויש אומרים ניכר שהוא מכוער הרק בתפלתו כאילו רק בפני המלך
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המשמיע קולו בתפלתו הרי זה מקטני
אמנה: אמר רב הונא לא שנו אלא שיכול לכוין את לבו בלחש אבל אין יכול לכוין את לבו בלחש מותר וה"מ ביחיד אבל בצבור אתי למיטרד צבורא:
He who makes his voice heard during prayer is of the small of faith; he who raises his voice when praying is of the prophets of falsehood; he who belches and yawns [during prayer] is of the proud of spirit; he who sneezes during prayer should regard it as a bad omen, and some say that it is an indication that he is contemptible; he who spits during prayer is as though he spits in the presence of the King !
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"He who makes his voice heard during prayer is of the small of faith." Rab Huna said : This teaching applies only to one who is able to direct his heart when whispering [the words of prayer] ; but if he is unable to do so, he is permitted [to pray aloud]. And this holds good only of one praying alone ; but with a congregation it would cause disturbance to others.
ומנא לן דבתפלה דתניא (דברים יא, יג) לאהבה את ה' אלהיכם ולעבדו בכל לבבכם איזו היא עבודה שהיא בלב הוי אומר זו תפלה
And from where do we know [that one should ask for rain] in Davening? Since we learnt in a Baraisa "To love Hashem your G-D, and to serve Him with all your heart" what's the service that is done with the heart? You must say davening.
(יז) אֲ֭דֹנָי שְׂפָתַ֣י תִּפְתָּ֑ח וּ֝פִ֗י יַגִּ֥יד תְּהִלָּתֶֽךָ׃
(17) O Lord, open my lips, and let my mouth declare Your praise.
Even better, our soliloquies prove useful. For example. when it comes to looking for something, saying the word out loud makes the thing easier to find, says Gary Lupyan, assistant professor of psychology at the University of Wisconsin, Madison. Saying what you're looking for out loud "helps you keep the visual representation of the object in mind better," says Lupyan, who wrote a paper on the topic. "The name helps you…visualize the object, enabling you to actually see it better."
Talking aloud also helps people solve problems. Chatting to yourself about how you'll answer interview questions for that dream job, or how you'll handle that serious talk with your boyfriend, prepares you for the actual experience. "Self-talk allows you to run through alternative scenarios and prepare alternative reactions and strategies," says Frank Ghinassi, vice president and director of the Department of Psychology at UPMC's Western Psychiatric Institute and Clinic.
-(http://www.today.com/health/talk-yourself-out-loud-here-s-why-experts-say-s-t76531)