Should a worshipper recite Surat Al-Fatiha and the following surahs aloud or silently if someone joins him during a loud prayer after the worshipper has already recited Surat Al-Fatiha silently?

For the Imam and the person praying alone, it is sunnah to recite loudly in audible prayers according to the Maliki and Shafi'i schools. Whereas, according to the Hanafi and Hanbali schools, the person praying alone has the choice between reciting loudly and reciting silently. If you are reciting loudly and someone joins you in prayer, do not change anything and continue your recitation. However, if you are reciting silently and someone joins you in prayer, do not repeat anything, but recite loudly for the remainder of the recitation to make the worshipper hear, as there is scholarly consensus on the Imam being required to recite loudly. It is sunnah to recite loudly in the two rak'ahs of Fajr, Maghrib, and Isha prayers, and in the Jumu'ah prayer, and to recite silently in Dhuhr and Asr prayers, and the third rak'ah of Maghrib, and the third and fourth rak'ahs of Isha.