A point was made a while ago that it may be an error with a particular batch (or batches) of covers. This being said, it may be prudent to at least check if your replacement cover was from the same batch or line as you old one...? It may even be a random
glitch in the manufacturing process of the covers that fixes the issue for some people.
Still encountering the issue here. I can further add that it does appear to more readily happen when I first touch the touch pad after doing something else for a while, so it may indicated an accumulated charge that when discharged resembles the "mute"
signal and the Surface reacts accordingly.
It also just recently occurred when I put my lapdesk with the Surface on it, open and standing with cover attached, onto a pillow with a drop of a few cm.
Thinking that I might just get the cover exchanged seeing as there doesn't appear to be much progress in here, even though there are posts stating that there have been no improvement. Maybe I'll get lucky with a latter batch or something that just happens
to not have the issue :-/
I replaced my cover about two weeks ago and I have not had the muting problem since, I'm just throwing this up here maybe it will help someone, but the batch number on my cover is 104481323951/ model #1515 / N1364 Z396