Reason he can't hear, probably:
1. Twilight style: ShE'S sPeCiAL
2. She just doesn't talk in her brain (because she's mute), probably thinking in 'feels'? pr anything other than 'words'.
For a commoner who raised her rank as knight and become the Emperor's bodyguard... this MC surely quite innocent.
Also, I'm 80-90% sure the Emperor knew... or maybe not?