All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We have already clarified in Fataawa 88878 and 82939 that a person who performs the supererogatory night prayers has the choice either to pray loudly or silently, and this is in regard to Al-Faatihah (the opening chapter of the Quran) and another chapter which one recites.
As regards the Tashahhud, glorifying Allaah during bowing or prostration positions, and other mention of Allaah, then this should be said silently. The jurists stated that this applies to all prayers, whether the night prayers or other prayers.
Allaah Knows best.