Mine has been on for a few hours now, it's a little warmer where you've made a circle than the opposite side, but I wouldn't call it warm.
(31c vs 27c on the opposite side)

Will try to remember to feel/measure tomorrow morning after it's been off all night.

The same for me. But right now I know what it happened. I took out me HDMI cable from my TV, and soundbar came to cool state. Previous day I turned off me TV, when something was playing on my Apple TV. I think, that this could made some issues, and the TV still send the signal to my soundbar.

I make some test, and let you know.

I had a look at it this morning, it´s the same temp all over, about 24c. 

My room temperature is 24c.

Your TV should turn off your Apple TV when it turns off and vice versa. So it seems strange, but you can check if your Apple TV turns off (the white light goes out)

But you are right, it could seem that your Arc Ultra was still receiving some kind of input, whether from the TV, Airplay or Bluetooth. (Maybe the SONOS App can reveal it)

I have disabled Touch control and have never activated voice control, I don't know if that has any effect.

You should also check if your speakers are up to date, as there has just been an update for the Sonos App and one for the Arc Ultra.

Could I ask you to measure the temperature in this place exactly? I want you to measure it after a few hours of watching TV and from a distance of 10 cm, for example. Thanks. I don't know if it matters, but my Soundbar works with two era 300 speakers. In my case, the temperature after 5 hours is about 38-39 degrees.

I mostly watch regular TV that doesn't play Dolby/Atmos and thus my Era300 doesn't activate.
I can measure temp after a few hours but as I said without Dolby/Atmos, otherwise it will be a day where I have a marathon of movies :)

Hi ​@addriano 

Thanks for your post!

This is entirely normal and nothing to worry about - Arc Ultra has a lot of processing power, and even at idle will produce a little heat. It may have been performing some task, such as indexing your Music Library, for example.

I hope this helps.

Hi everyone,

I think the problem has been solved. It was the TV. If any of you have a Sony TV, in the sound settings there is something like eARC auto or off. If you have this set to auto, the TV can wake up the soundbar. I set this to off and the problem disappeared. The soundbar is now cool in the morning. It also doesn't go into mute mode and the green light doesn't come on by itself.

Hi ​@addriano 

Glad to hear you have found a workaround. Please be aware that by disabling eARC mode, you will be restricting your Arc Ultra to lossy codecs only, meaning lower quality (for TV audio). If you do not use Blu-rays or game consoles, this will likely not be an issue for you, however.

I hope this helps.

And this is just the strangest thing. For example, when I was watching TV through my cable TV set-top box, I had the eARC setting on the soundbar set to 'Auto'. This gave me stereo PCM 2.0 sound, but when I switched it to 'off', I had Dolby Digital Stereo 2.0. I think the sound is better with the second one.

Hi ​@addriano 

This is probably due to the Digital Audio Output Format setting on your TV - setting it to Passthrough or Bitstream is usually best (though doing so will likely set you back to PCM again, but that probably will have been the original format used with the TV broadcast).

I hope this helps.