Should one make up a missed rak'ah audibly or silently in a congregational prayer that is recited audibly?

The correct view is that what a latecomer catches up with the Imam is considered the beginning of his prayer, based on the Prophet's (peace and blessings be upon him) saying: "(So) whatever you catch up with, pray, and whatever you miss, complete." The meaning of "complete" is to perfect. Thus, if a latecomer catches the second unit (rak'ah) of Maghrib prayer with the Imam, it is considered his first unit. Then, the Imam's third unit becomes the second for the congregant. After the Imam's salutations, the congregant stands to pray his third unit, reciting Surat Al-Fatihah silently in it.