小さな声でも話すな。まして、大きな声で話すなど、もってのほかだ。

。、、。

Don't even whisper. Speaking loudly is absolutely out of the question, let alone that.

Literal

Small voice [even with-でも] speak [prohibitive-な]. Much-less, loud voice [with-で] speak [such as-など] outrageous [copula-だ].

A two-part escalation. Even quiet speech is forbidden (prohibitive な), and loud speech is もってのほか ('outrageous'). まして ('much less, let alone') marks the escalation from mild to extreme. など has a dismissive tone, treating loud speech as beneath consideration.