I have a new Sonos Arc (configured as a home theater system with 2x Era 100 and Sub Mini). Firmware up to date. TV connected to Sonos via HDMI ARC. 

After muting - whether via the TV remote or the app - after about 10 seconds Sonos turns the sound on anyway. The situation does not occur when the sound is played via the Sonos app or AirPlay. 

Turning off the “Auto Play” option did not change anything.