We learn many essential halakhot from the prayer of Ĥana, who stood and begged God to remember her and grant her a son. Her prayer was accepted and she merited giving birth to Shmuel the prophet, who was the greatest prophet of Israel behind Moshe. The verse states: “Ĥana spoke to her heart, only her lips moved, and her voice was not heard” (1 Shmuel 1:13). The Sages teach: “‘Spoke to her heart’ – from here [we learn] that the person praying must focus his heart. ‘Only her lips moved’ – from here [we learn] that the person praying must articulate the words. ‘Her voice was not heard’ – from here [we learn] that it is prohibited to raise one’s voice while praying” (Berakhot 31a).
The idea behind the Amida prayer is to express the soul’s deepest aspirations before God, and therefore it is not proper to recite the words aloud and to disclose them outwardly. On the other hand, one does not fulfill her obligation with thought alone, since every idea must possess some tangible expression in this world. Very often, our inner desires are praiseworthy, but their outer expression is flawed. Therefore, our task is to express our inner good will outwardly, thereby repairing the world. Hence, even the subtlest mitzva like prayer, requires some sort of expression – the silent articulation of the words with one’s lips.
There are different practices regarding the proper way to pray silently. According to most poskim and a few kabbalists, one reciting the Amida must utter the words in such a way that only she hears her voice but those praying next to her do not (SA 101:2; MB 5-6). According to most kabbalists, the Amida is so intense and internal that one should not even hear her own words; she should only mouth the letters with her lips (Kaf Ha-ĥayim 101:8). The woman praying may choose the way which enables her to have the most kavana in her prayer.
Be-di’avad, even if one recites the Amida out loud, she fulfills her obligation. Therefore, if one who has difficulty concentrating silently is praying alone, in a place where she will not disturb the prayer of others, she may pray aloud. However, even in that situation, she should not raise her voice, for one who does so behaves like the false prophets who think that their gods are hard of hearing and that one must yell in order to be heard (Berakhot 24b).
On the Days of Awe, there are parts of the Amida where it is customary to raise one’s voice slightly, and one need not be concerned that she is disturbing others around her because on those days everyone has maĥzorim (SA 101:3). Nevertheless, even in those places, one who wishes to enhance the mitzva prays silently.
Regarding the other sections of the prayer service, such as Birkhot Keri’at Shema and Pesukei De-zimra, which are not as inward-focused and intense as the Amida, all agree that the person praying must hear the words she is reciting. Be-di’avad, if she only mouthed the words without hearing them, she fulfilled her obligation, but if she thought the words in her mind, without even mouthing them, she did not fulfill her obligation.