The Microphone signal come from MSX as a square wave and it is attenuated and filtered down to  20mVpp.

The noise on the output signal is due to a combination of noisy probes and an oscilloscope reading signals at its maximum sensitivity. Tried 3 different probes to verify that.

It is worth to notice that this circuit should be properly shielded, otherwise the output signal can get distorted, so the cable that connects the MSX to the box must provide the shield connections.