אורח חיים ק״אOrach Chayim 101

Se’if 1א׳

הַמִּתְפַּלֵּל צָרִיךְ שֶׁיְּכַוֵּן בְּכָל הַבְּרָכוֹת, וְאִם אֵינוֹ יָכוֹל לְכַוֵּן בְּכֻלָּם, לְפָחוֹת יְכַוֵּן בְּאָבוֹת; אִם לֹא כִּוֵּן בְּאָבוֹת, אַף עַל פִּי שֶׁכִּוֵּן בְּכָל הַשְּׁאָר, יַחֲזֹר וְיִתְפַּלֵּל. הַגָּה: וְהָאִדָּנָא אֵין חוֹזְרִין בִּשְׁבִיל חֶסְרוֹן כַּוָּנָה, שֶׁאַף בַּחֲזָרָה קָרוֹב הוּא שֶׁלֹּא יְכַוֵּן, אִם כֵּן לָמָּה יַחֲזוֹר (טוּר).

That One Must Direct One's Intention For All The Blessings and That One Can Pray In Any Language, containing 4 s'ifim. One who prays [the Amidah] needs to direct one's intention for all the blessings, but if one is not able to direct one's intention during all of them, one should at least concentrate on the blessing of the forefathers [the first blessing of the Amidah]. If one didn't direct one's intention while saying the blessing of the forefathers, even if one directed one's intention for the rest [of the blessings], one needs to return [to the beginning of the Amidah] and pray. Gloss: (Nowadays we do not return [to repeat the prayer when it is] due to a lack of intention, because [there's a fair chance that] even in the repeating [of the Amidah] it is likely that one will not direct one's intention [again]; if so, why should one return?) (Tur)

Se’if 2ב׳

וְּלֹא יִתְפַּלֵּל בְּלִבּוֹ לְבַד, אֶלָּא מְחַתֵּךְ הַדְּבָרִים בִּשְׂפָתָיו וּמַשְׁמִיעַ לְאָזְנָיו בְּלַחַשׁ, וְלֹא יַשְׁמִיעַ קוֹלוֹ; וְאִם אֵינוֹ יָכוֹל לְכַוֵּן בְּלַחַשׁ, מֻתָּר לְהַגְבִּיהַּ קוֹלוֹ; וְהָנֵי מִלֵּי בֵּינוֹ לְבֵין עַצְמוֹ, אֲבָל בְּצִבּוּר, אָסוּר, דְּאָתֵי לְמִטְרָד צִבּוּרָא. הַגָּה: וְאִם מַשְׁמִיעַ קוֹלוֹ בְּבֵיתוֹ כְּשֶׁמִּתְפַּלֵּל, כְּדֵי שֶׁיִּלְמְדוּ מִמֶּנּוּ בְּנֵי בֵיתוֹ, מֻתָּר (טוּר).

And one should not only pray in one's heart, but one must actually enunciate the words with one lips and let them be heard in a whisper tone by one's own ears, [however] one should not let one's voice be heard [fully during the Amidah]. [However,] if one is unable to direct one's intention [to the Amidah] while in a whisper tone, one is allowed to raise one's voice. And this is [only] when one is praying privately, but [if one is praying] with the congregation, it is forbidden, as this is a nuisance for the congregation. Gloss: If one is letting one's voice be heard in one's home when praying so that the members of one's household will learn from one, it is permitted (Tur).

Se’if 3ג׳

יֵשׁ אוֹמְרִים שֶׁבְּרֹאשׁ הַשָּׁנָה וְיוֹם כִּפּוּר מֻתָּר לְהַשְׁמִיעַ קוֹלָם בַּתְּפִלָּה, אֲפִלּוּ בְּצִבּוּר. הַגָּה: וְכֵן נוֹהֲגִין, וּמִכָּל מָקוֹם יִזָּהֲרוּ שֶׁלֹּא לְהַגְבִּיהַּ קוֹלָם יוֹתֵר מִדַּאי (דְּרָשׁוֹת מהרי"ו).

Some say that on Rosh Hashanah and Yom Kippur it is permitted to let one's voice be heard in one's prayer, even in public. Gloss: And this is what we practice; however, people must be careful not to raise their voice too much (D'rashot Mahari"v [Ya-akov Weil])