How do I remove electrical interference when using same source power?
The interference is probably not from a ground loop, but from the mic picking up EMI from the raspberry pi or the DC to DC converter. Put a inductive ferrrite cable clamp on all cables going to the mic receiver to see if it is conducted emissions (EMI going into the cables and then to the mic)
If it isn't conducted emissions, then it could be EMI/radio emitted by the PI or the DC DC converter going through the air. You would then need to shield the MIC with a faraday cage that could block radio emissions and/or move the Pi and converter away from the mic.