The M1 DC microphone has a small potentiometer to set the gain of the microphone. It is possible that this is set too high. Look for a small hole in the mic casing with a white cap containing a screwdriver slot beneath the hole. It will not do any harm to rotate this anticlockwise a small bit at a time to see if it improves things. Make a note of the original position. It would obviously be better if someone could do this with an oscilloscope to measure the audio level.
I used to repair lots of DC10-30s and had no trouble with them in PA28s. I prefer the 10-13.4