I bought a remote speaker-microphone (RSM) for a DMR portable from eBay (~$12 posted). Experience says that these suffer RF ingress which is seriously bad in DMR due to the amplitude modulation of the transmitted signal.
This RSM had somewhat improved filtering around the electret compared to others I have purchased.
Above, there is cap of unknown value soldered right across the electret terminals (top middle). The track leading to the electret has been prepared to install a series inductor to further reduce RF ingress.
The fix involved cleaning the mask off +ve track and cutting it to accept a SMD choke (10µH) right near the electret pin.
Above, a 10µH inductor soldered across the cut track.