Is it prescribed for me to raise my voice in recitation during the 'Isha prayer after someone joined me to pray in congregation, knowing that I had been praying it silently, and should I restart reciting Al-Fatihah or continue from where I left off?

Reciting aloud in prayer is a Sunnah for the Imam, while an individual praying alone has the choice between reciting aloud or silently. Reciting aloud in a prayer that typically involves audible recitation is not obligatory; rather, it is preferable. So, if a person recites silently in a place where audible recitation is customary, their prayer is valid, but it is incomplete. If someone joins you while you are praying alone, it is Sunnah to recite aloud because you have become an Imam, and you are not required to repeat Al-Fatihah. If you do not recite aloud until you have completed Al-Fatihah or the entire bowing (rak'ah), or if you do not recite aloud at all, there is no harm, and your prayer is valid.