Muhammad Al-Musawi Al-Amili writes:

قوله ويجب الجهر بالحمد والسورة في الصبح وفي الأوليين من المغرب والعشاء، والإخفات في الظهرين وثالثة المغرب والأخيرتين من العشاء هذا هو المشهور بين الأصحاب ونقل فيه الشيخ في الخلاف الاجماع وقال المرتضى رحمه الله في المصباح: إن ذلك من السنن الأكيدة وقال ابن الجنيد: لو جهر بالقراءة فيما يخافت بها أو خافت فيما يجهر بها جاز ذلك، والاستحباب أن لا يفعله

,,As for his statement: ﴾One must recite the chapter of Al-Fatihah and another in prayer aloud in the morning (Fajr) and in the first two prostrations (Rak’ah) in the evening (Maghrib) and at night (Isha’) and silently at noon (Zuhr) and afternoon (Asr) and in the third (Rak’ah) in the evening (Maghrib).﴿ Thus, this is the most widespread among the Companions and At-Tusi reported the consensus (Ijma’) about it in the Book of Al-Khilaf and Al-Murtada, May God bless him, said in the Book of Al-Misbah: ﴾This is belongs to the customs (Sunnah), which is to be taken seriously.﴿ Ibn-ul-Junaid said: ﴾If one recited loudly what one is required to recite softly, or recited softly what one is required to recite loudly, it is permissible, but it is desirable (Mustahab) to refrain from doing so.﴿” [Madarik-ul-Ahkam, Volume 3, page 356]

احتج الشيخ بما رواه عن زرارة، عن أبي جعفر عليه السلام، قال قلت له: رجل جهر بالقراءة فيما لا ينبغي أن يجهر فيه، أو أخفى فيما لا ينبغي الإخفاء فيه، فقال: أي ذلك فعل متعمدا فقد نقض صلاته وعليه الإعادة، فإن فعل ذلك ناسيا أو ساهيا أو لا يدري فلا شئ عليه وجه الدلالة: قوله عليه السلام أي ذلك فعل متعمدا فقد نقض صلاته وعليه الإعادة فإن نقض الصلاة بالضاد المعجمة كناية عن البطلان، والإعادة إنما تثبت مع اشتمال الأولى على نوع من الخلل واحتج الشهيد في الذكرى على الوجوب أيضا بفعل النبي صلى الله عليه وآله، والتأسي به واجب وهو ضعيف جدا فإن التأسي فيما لا يعلم وجهه مستحب لا واجب، كما قرر في محله

,,At-Tusi presented as evidence, what Zurarah reported from Muhammad Al-Baqir (a.) who recited to him: ﴾Someone recited aloud, that which should not be recited aloud or recited softly, that which should not be recited softly.﴿ He said: ﴾If he did that intentionally, he has violated the prayer and it is obligatory on him to repeat it and if he did that out of forgetfulness or carelessness, then nothing is obligatory upon him.﴿ The viewpoint of proof consists in the statement of the Imam (a.): ﴾If he did that intentionally, he has violated the prayer.﴿ For the violation of the prayer is a metaphor for invalidity and its repeating comes into effect, only if there is some form of blemish and Ash-Shahid Ath-Thani in the book Adh-Dhikra put forward as proof of obligation the action of the Prophet (s. ) and that his imitation is binding, but this is very weak, because imitation in matters, whose background is unknown, is indeed desirable (Mustahab), but not obligatory, as it has been confirmed in the appropriate place.” [Madarik-ul-Ahkam, Volume 3, page 356 – 357.]

احتج القائلون بالاستحباب بأصالة البراءة من الوجوب، وقوله تعالى: ولا تجهر بصلاتك ولا تخافت بها وابتغ بين ذلك سبيلا وجه الدلالة: أن النهي لا يجوز تعلقه بحقيقة الجهر والإخفات لامتناع انفكاك الصوت عنهما. بل المراد والله أعلم ما ورد عن الصادق عليه السلام في تفسير الآية وهو تعلق النهي بالجهر العالي الزائد عن المعتاد، والإخفات الكثير الذي يقصر عن الأسماع، والأمر بالقراءة المتوسطة بين الأمرين، وهو شامل للصلوات كلها

,,Those, who believe therein, that it is desirable (Mustahab), put forward as proof that from the basic assumption (Asl) there is no obligation, as well as the verse: ﴾Do not be loud in your prayer and do not be quiet in it, but seek a way in between.﴿ (17:110) The viewpoint of proof consists, that the prohibition does not really refer to the loud and quiet tone, since the voice cannot deviate from both, but the meaning is that, – and God knows best – which was narrated by Ja’far As-Sadiq (a. ) as an interpretation for this verse and namely, that the prohibition refers to a very high volume that exceeds the normal level and a very low volume, that is hardly perceived by the hearing and it is commanded to recite in a moderate tone in between and this applies to all prayers.” [Madarik-ul-Ahkam, Volume 3, page 357]

وما رواه علي بن جعفر الصحيح، عن أخيه موسى عليه السلام، قال: سألته عن الرجل يصلي من الفرائض ما يجهر فيه بالقراءة، هل عليه أن لا يجهر؟ قال: إن شاء جهر وإن شاء لم يجهر وأجاب عنها الشيخ بالحمل على التقية لموافقتها لمذهب العامة قال المصنف رحمه الله: وهو تحكم من الشيخ، فإن بعض الأصحاب لا يرى وجوب الجهر بل يستحبه مؤكدا والتحقيق أنه يمكن الجمع بين الخبرين بحمل الأول على الاستحباب أو حمل الثاني على التقية ولعل الأول أرجح لأن الثانية أوضح سندا وأظهر دلالة، مع اعتضادها بالأصل وظاهر القرآن

,,As for the tradition, that Ali Ibn Ja’far authentically (Sahih) reported from his brother Musa Al-Kazim (a.): ﴾I asked him about someone, who performs the obligatory prayers, wherein one recites aloud, whether he may recite softly, to which he said: ,,If he wished, he recited loudly and if he wished, he recited quietly.”﴿ So At-Tusi replied to this by saying, that he understands it to be said out of cautiousness (Taqiyyah), because it agrees with the majority. Ja’far Al-Hilli, May God bless him, said: ﴾It is At-Tusi’s own judgment, because some companions do not consider reciting loudly as an obligation, but consider it as highly desirable (Mustahab).﴿ The truth is that both traditions are compatible with each other by taking the first one as desirable (Mustahab) or the second one as said out of cautiousness (Taqiyyah) and the first one may be more applicable, because the second tradition has a clearer chain and contains a stronger hint. Apart from that, it supports the basic assumption (Asl) as well as the outer wording of the Qur’an.” [Madarik-ul-Ahkam, Volume 3, page 357 – 358]