Our second M20d failed in this manner during our church service this last Sunday. We suddenly lost all sound output while instrumentalists were playing (keyboard and cello). I tried cycling the "Mute All" button with no reaction. Turned power off and back on and everything worked again. I will be opening a support ticket tomorrow after I obtain the serial number. This is a new unit, received in March as a replacement for a defective M20d but not used for church services until late July.