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Automatic lowering one signal when it is triggered by another signal can be done with the 'Ducker' module. 
 
The ducker is used to attenuate one channel of audio when the audio input of the 'priority' channel (in this case the microphone) reaches a specified level or threshold.
 
See also attached image as example. 


 
For an explanation of the settings in the ducker please refer you to the manual of the DM8000.