As an imam leading the fajr/maghrib/isha prayer, is it wajib or sunnah to recite out loud (vs silently)?

It is more than standard for the imam to recite out loud during the first two rakat of fajr, maghrib and isha prayer. However what is the actual ruling of this practice - wajib or mustahab? In other words, if the imam recites silently then did he actually commit a sin/mistake?