1. It is obligatory for a man to recite al-Fātiḥa (Ḥamd) and the surah of the first two rakʿahs of the morning (Ṣubḥ), Maghrib, and ʿIshāʾ prayers with jahr (aloud).
And it is obligatory for both men and women to recite al-Fātiḥa and the surah of the Ẓuhr and ʿAṣr prayers with ikhfāt (silently).

Ayatollah Sistani: It is obligatory — as an obligatory precaution — for a man to recite al-Fātiḥa and the surah of the morning, Maghrib, and ʿIshāʾ prayers aloud; and it is obligatory — as an obligatory precaution — for both men and women to recite al-Fātiḥa and the surah of the Ẓuhr and ʿAṣr prayers silently.

2. A woman may recite al-Fātiḥa and the surah of the morning, Maghrib, and ʿIshāʾ prayers either aloud or silently, but if a non-maḥram (stranger male) hears her voice, it is better for her to recite silently.

Ayatollah Sistani: If a non-maḥram hears her voice in a situation where making the voice heard to a non-maḥram is ḥarām, she must recite silently. If she deliberately recites aloud, then as an obligatory precaution, her prayer is invalid.

Ayatollah Makarem Shirazi: If non-maḥrams hear their voice, the recommended precaution is that they should recite silently.

3. If one chooses to recite al-Fātiḥa, then as a precaution, “Bismillāh” should also be recited silently.

Ayatollah Sistani: If one recites al-Fātiḥa in the third and fourth rakʿahs, it is not obligatory to recite “Bismillāh” silently, except when acting as a follower (maʾmūm); in that case, the obligatory precaution is to recite “Bismillāh” silently as well.

Ayatollah Makarem Shirazi: It is obligatory in the third and fourth rakʿahs to recite either al-Fātiḥa or the tasbīḥāt al-arbaʿah silently, even “Bismillāh” (as an obligatory precaution).

4. The obligation of reciting aloud or silently in the first and second rakʿahs of prayers applies only to al-Fātiḥa and the surah. The obligation of reciting silently in the third and fourth rakʿahs applies only to al-Fātiḥa or the tasbīḥāt al-arbaʿah. As for other adhkār (remembrances) in daily prayers — such as the dhikr in bowing (rukūʿ), prostration (sujūd), tashahhud, salām, and other remembrances — the mukallaf (legally responsible person) has the choice to recite them either aloud or silently.

5. There is no difference between prayers performed on time (adāʾ) and those performed as make-up (qaḍāʾ) with regard to the obligation of jahr and ikhfāt, even if the qaḍāʾ prayer is performed out of precaution.

6. The criterion of jahr is to make the essence of the voice apparent. Ikhfāt means not making the essence of the voice apparent — even if someone standing next to the worshipper hears him.

7. If a person deliberately recites silently where he must recite aloud, or recites aloud where he must recite silently, his prayer is invalid. But if this occurs due to forgetfulness or ignorance of the ruling, his prayer is valid. Moreover, if he realizes during the recitation of al-Fātiḥa, the surah, or the tasbīḥāt that he has been reciting in the wrong mode (aloud or silent), it is not necessary to repeat what he has already recited incorrectly.