This happens to me as well but I do not see it as an issue because when I turn the tv on or stream audio to it, the system automatically unmutes or a simple volume tap will unmute it.

I know, but those green LEDs annoy me 

I use Sonos Voice Control to turn on the TV.  Just say “Hey Sonos. Turn on TV”.  If you don’t like the green light turn off the display “Status Light” in the app.

If you don’t like the green light turn off the display “Status Light” in the app

Unfortunately, the display's "Status Light" is turned off, which has no effect on the mute green light :( 

I would use a small dot of black tape.

And again a miss. I have a white Arc 

Too bad. ‘White’ complicates your issue because white tape may not be opaque enough and there will probably be slight differences between the white tape sheen and the ARC finish. You may need a two layer ‘dot’. A black dot covered by a white dot that exactly matches the color shade and sheen of the ARC.

Another approach would be covering the light with a decorative, opaque  jewel of some sort.

Even if SONOS decides to change this status light behavior, I wouldn’t expect expect this to be done soon. Tape or jewel could be applied today.

I found some rather effective white electrician’s tape at my local hardware store, it works quite well, and is fully opaque. Unless I’m looking for it, it really is not seen. 

Hi everyone,

I think we've figured out the problem. It turns out the issue was with the TV. If you've got a Sony TV, there's something called eARC “Auto” or “off” in the sound settings. If you have this setting on “Auto”, the TV can wake up the soundbar. I switched this setting “off” and the problem went away. The TV doesn't put it into a mute state and the green light is no longer on by itself.

For others reading this thread, there can be challenges in turning off eARC, as explained by the Sonos personnel in the other thread.