After sitting idle for over a week, my M1001 was muted again on power up. The display was working fine, and a station was apparently playing OK, but no sound through the analog outputs. Curiously, merely unplugging the DC power plug from the M1001 did not solve the problem. I had to unplug the AC adapter and plug it in again before it started working.
I am using 2.7.78 firmware, and thought we left those problems behind with the last upgrade, but apparently not. This is how the problem occurred for me before, but then I only had to leave it off overnight for the mute occur, while now it seems a week or more is required, so I suppose that is progress of sorts.