Should a person recite Qur'an out loud when another person joins him in Salah?
You should recite loud enough so the person offering Salah with you can hear you, whether you offer Nafilah (supererogatory prayer) or Faridah (obligatory prayer) at night, if there is someone else offering Salah with you; but if there is no one praying with you, it is permissible for you
to make subvocal or loud recitation. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.