Peace be upon you.
Ayatollah al-Uzma Sayyid Sistani (may Allah protect him):
1) It is obligatory to recite quietly, as a precaution.
2) Regarding missed prayers, if one recited aloud out of ignorance or forgetfulness, their prayer is valid. If they were aware of the ruling or remembered during the recitation, what has passed is valid, and they should follow their duty for the remainder.
3) You may rely on the imam's recitation.
Note: It is obligatory for the follower to refrain from reciting in the first two rak'ahs of the noon prayers, and it is recommended to engage in glorification or other forms of remembrance. They must refrain from reciting in the dawn prayer and in the first two rak'ahs of the evening prayers if they hear the imam's voice, even if it is just a murmur. In such cases, it is preferable to listen to the imam's recitation, although engaging in remembrance is not contradictory. If they do not hear anything from the recitation or murmuring, they have the option to recite quietly or refrain, with recitation being better.
This applies when the imam is in the first or second rak'ah of their prayer. However, if the imam is in the third or fourth rak'ah, the follower must act according to their duty. If they are in the first or second rak'ah, they must recite; if in the third or fourth, they may choose between recitation and glorification, with glorification being preferable. There is no difference in other forms of remembrance whether the prayer is performed in congregation or individually.
Ayatollah al-Uzma Sayyid Khamenei (may Allah protect him):
1) It is obligatory to recite quietly.
2) Regarding missed prayers, if one recited aloud by mistake, their prayer is valid.
3) You may rely on the imam's recitation.
Note: In the dawn prayer and the first two rak'ahs of the evening prayers, if the follower hears the imam's recitation of Al-Fatiha and the surah, even if it is just a murmur, they are not allowed to recite Al-Fatiha and the surah. It is also obligatory to refrain from reciting if they hear any of the words. However, if they do not hear the imam's voice, it is recommended to recite Al-Fatiha and the surah quietly, and if they recite aloud by mistake, their prayer is valid.
May you be successful in all good.