Should Sunnah prayers (Fajr, Maghrib, Isha) be recited aloud when performed in congregation?

One does not vocalize in the prescribed supererogatory prayers—whether at night or during the day—regardless of whether the worshiper is an imam or praying alone. Vocalization is performed in the absolute supererogatory prayers of the night if one is an imam, and a moderate vocalization if praying alone. As for the follower, they do not vocalize at all.