Try turning off HDMI CEC under Settings > Display & Sounds on the Fire TV Cube.

Thanks.  It was “on” and I shut it off.  Working so far.

Tonight the sound cut off on the Sonos ARC.  Unplugging the Amazon Fire TV Cube immediately restored the sound.  So, shutting off the CEC HDMI on the cube is not the solution.

Have you tried connecting the Fire TV Cube to a different HDMI port on the TV?

Not sure at this point.  I’m about to start using the HDFury Arcana.  So, I have a Roofull HDMI switch on its way.  The Fire TV will be directly plugged into a port on the Roofull, which will feed the Sonos ARC and the TV.  That would be the same as changing the TV input, I would think.

Sambucus, I am having the same issue. Gone from a beam via optical to beam gen 2 HDMI earc, it worked for a few days and now having the same issue as you. I’ve tried different HDMI ports and still does the same. 

Sambucus, after playing about with the Fire Tv, I was getting stumped. It seemed to be the issues started when the tv was off and couldn’t reconnect to the beam after powering the tv on. This was the same for the fire stick plugged into the mains. I changed how I powered the fire stick  by plugging the usb into the tv 5v USB port and it works perfect now. No audio issues or disconnecting from the beam.