Reciting out Loud in the Sunnah or Nafl Prayers
What is the ruling of reciting out loud or silently in the Sunnah (nafl) prayers?
In the name of Allah, we praise Him, seek His help and ask for His forgiveness. Whoever Allah guides none can misguide, and whoever He allows to fall astray, none can guide them aright.
It is preferred to recite quietly if the nafl prayer is offered by day, following the ruling on the daytime prayers. He has the choice between reciting out loud or silently in the night prayers, if he is praying alone, but reciting out loud is better, provided that it does not disturb others.
In case one is leading others in the nafl or witr prayer, then the imam (leader of prayer) should recite out loud so that those who are praying behind him can hear him.
For more details, watch this interesting talk with Sheikh Ibrahim Zidan.
Source: Huda Youtube Channel.