In order to perform the requested changes please log in to the web SMC web interface and go to System > Devices and under ViewPoint 8600/ VP9600/ Cloud Series MCUs you have the option Microphone mute control and make sure it is set to Disable. Make sure that the wanted MCU is used as the active MCU for resources allocation (active one)
Afterwards you can schedule a conference again and check the participant status to see if they have been unmuted.