And why is Witr read aloud in Ramadhaan?
The Witr Salaat in Jamaat succeeds/follows the Taraweeh Salaat in Jamaat, hence, it is restricted to the month of Ramadhaan only. (Ahsanul Fatawa vol.3 pg.455)
This is possibly the same reason why it is performed aloud.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai