Audible & Inaudible Salat
Is it halal for to offer the loud salat (such as fajr) in silence?
The minimum degree of audibleness is to hear your self and for the person next to you to hear you if you imagine one to be there. And there is no maximum limit for audibleness.
It is also one of the stressed sunnahs of the prayer for which prostrations of heedlessness (sajdat as-sahw) should be made for before concluding the prayer with the salam.
Shaykh Abdullah Bin Hamid Ali