System issue

I have only seen this on my 300 Pair, not on my Play 5 gen 2 or Play.

You’re right, the Atmos devices are the ones exposed to the issue, as they try to reproduce different formats. My Move 2 has no problems because it doesn’t try to play Atmos, even if the song is available in this format. 
Waiting time then, hoping Sonos find a quick solution.

Guess I had it backwards.  I’ll have to test a mix on my Atmos stuff.

That is my suspicion, too. At DE community there also is a user reporting that issue with Amazon Music. He uses a stereo pair of Era 300 and has that problem randomly. 
But… it never occurs with „3D Music“ option disabled in Era 300 settings. 
I tried to reproduce it with Amazon Music and a single Era 300 setup (3D Music enabled). But wasn’t able to reproduce it. 
Now i‘m going to try the songs mentioned above. 

Ok, now I was able to reproduce it with the mentioned Dire Straits album and can confirm, that it is an issue of audio format switching. 
As already said, it occurs if…

• you use an atmos compatible Sonos Speaker setup
• in room settings „3D music“ is activated 
• you play certain stereo tracks and the following track is an atmos track

In my case I played track 5 in HD and during the last 10 seconds volume increased. I paused playback, scrubbed some seconds back and resumed track 5. Now, though it’s a stereo track, there was the atmos badge. So obviously during the last seconds the format already switched to the following atmos format of track 6 and that caused the volume increase. 
 

Maybe that’s why Atmos seems so much louder than other format tracks.  Maybe the “Look Ahead” is seeing the Dolby Atmos tag and raising the volume of the HD track ahead of time.  Wonder if that’s an attempt to fix the not so perfect gapless track transitions when playing Dolby Atmos music.  I’ll test this later.  It’s Indy 500 now.

EDIT:  Nope.  The gapless track transition when playing Stravinsky: The Rite of Spring ( Dolby Atmos ) still has a glitch.  It is most noticeable at the transition from track 2 to track 3.  So that does not explain why a transition from a HD track to a Dolby Atmos track would raise the volume at the end of the HD track.

My personal suspicion is, that Sonos might have integrated something like a level adjustment/ normalization for atmos music tracks. 
Because in the past imo atmos music tracks had a much lower volume than stereo tracks at the same volume setting. 
So perhaps there’s a bug and the adjustments already starts when the format of the next track is recognized instead of when it starts playing. 
 

If that’s the case then it’s been that way for a while.  In my case Atmos has always been louder.

Ran a test with Amazon Music Unlimited where I started a HD track then while it was playing I added a Dolby Atmos track using Play Next.  The volume increased a lot at 16 seconds from the end of the HD track.  Good I read this subject prior to this test.  Because I did read about this problem I kept the volume pretty low for the test.

Sonos needs to address this before someone blows up their system.

Are you talking about atmos music in Sonos music mode? When Sonos started supporting atmos music I always had to rise volume level of atmos music tracks to get the same result as playing stereo music. 
After whichever update that changed, but as said, I suppose that could have been done by a level adjustment. And perhaps this isn’t working correctly at the moment.