Assalamualaikum ustaz. What is the status of a person who followed an imam who recited in a low voice for Maghrib prayer?
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions and all those who follow his footsteps until the Last Day.
Reciting Surah al-Fatihah loudly for Maghrib, Isya and Subuh is sunnah and is included as one of the hai’ah sunnah (الهيئة).  If a person, regardless of whether he is the makmum or imam, forgets or did not recite surah al-Fatihah (or other surahs after surah al-Fatihah) loudly in prayers where it is sunnah to be recited loudly, then it is makruh for he has left a sunnah matter. However, it doesn’t make the prayer invalid.
Imam al-Syirazi explained this matter saying that by reciting the recitations in Maghrib, Isyak and Subuh prayers is among the sunnah for an imam. He said: “It is sunnah for an imam to recite loudly the recitations for Subuh, two first rakaat of Maghrob prayer and 2 first rakaat for Isyak prayer. The evidence is the citations from the khalaf and salaf scholars.” 
However, such an act isn’t considered as contradicting the sunnah of the Prophet PBUH for the Prophet PBUH instructed us to pray just as he prayed. The Prophet PBUH said:
وَصَلُّوا كَمَا رَأَيْتُمُونِي أُصَلِّي
“Pray as you have seen me pray,” 
We would like to state that if a person followed an imam who recite the recitations for Maghrib, Isyak and Subuh prayer in a low voice, then the prayer is valid and he doesn’t have to repeat the prayer for increasing one’s voice for these prayers (Maghrib, Isyak and SUbuh prayers) is just sunnah as previously stated above.
Thus, a faqih imam who understands the Fiqh al-Solah properly will take every step necessary to perform prayer in line with the shariah. We end with the supplication:
اللَّهُمَّ أَعِنَّا عَلَى ذِكْرِكَ ، وَشُكْرِكَ ، وَحُسْنِ عِبَادَتِكَ
“O Allah, help me to remember You, to give You thanks, and to perform Your worship in the best manner.”