Capacitor C21 is connected at the wrong side of resistor R7.
The error was made while reverse enginering the original box.
This capacitor is in the original box (ET312B) connected to the input jack.
The MIC input of the MK-312BT is now considerably muted by C20.
Easiest solution is to solder the left leg of C21 to the other side of R7 on the component side.