I think it could just be the difference between the way Sonos detects and plays “TV” audio as opposed to “Music” audio. When you play music from your Apple TV’s apps, you are still in “TV” mode.

But you are supposed to get stereo PCM audio from Spotify and Plex. I don’t believe they offer 5.1 mixes of any of the music they stream. Now whether or not the stereo signal should still be playing out of your surround speakers is another question.

I just played music from the Music app from an Apple TV (3rd Gen) and the audio is clearly coming out of my surround speakers too.

If you change the Audio Format setting on the Apple TV, does it make a difference?

Plex and Spotify are only set for stereo output but my assumption was the same as yours that Sonos should use all the speakers in the group for this output, which they are not. 
 

I tried the Apple Music app as well and get the same, only the arc and sub are playing. I tried changing the ATV audio setting and still no difference. 
 

so there is an issue that the surround speakers are not playing the music. Not sure where to start changing setting to see how to resolve the issue. 

You won’t get your surrounds working from an Apple TV device connected to your tv I believe when playing music. I don’t know about apple accounts but with my Amazon prime account I added Amazon’s music service in the Sonos app and can play music through that which plays through the surrounds as well. You want to see if your apple account will let you configure Apple Music in the Sonos app. I would suspect it will. 

@Mooneymdp I just tested the Spotify app on my Roku Ultra and could clearly hear audio coming from my surround speakers. If you increase the TV Level of the surrounds in the Sonos app, does it make a difference?

You sure you aren’t just getting some upsampling? If I put a cd in my blurry player some songs will get some upsampling. It’s not the same as playing the same thing through the app.

Whatever audio I’m getting out of my surrounds sounds good and makes my music listening experience more immersive. I experience the same from my surrounds listening to my purchased music from the Music app on my old Apple TV (3rd Gen). The OP is currently getting nothing out of his surrounds. We think he should be getting something.

Perhaps. It’s not my understanding of how devices attached to TVs work with Sonos sound bars. Stereo won’t get much from the surrounds except some upsampling. If it’s not just stereo that would be different.

As I mentioned, if the OPs apple account allows it,setting up Apple Music in the Sonos app would be better. 

When I use the music service on Sonos app the music plays through all speakers as it should.  This is only when I am playing from the apps directly on the ATV.  The reason I use these apps is because the Plex picks up music from my own library and I use Spotify for streaming.  it is also a nice thing to have the 4K ATV screen savers playing in the backdrop or use the lyrics from the app.  I can switch to to service directly on the Sonos app but would loose those features.

I am just looking to confirm first if the music via an app on the ATV should be playing through all speakers, if this is not expected behaviour I can move on with the limitation.  However if it should play on all speakers i will spend more time trying to figure out how it works.  I have played about with the surround speaker level with no difference.

I have a Nvidia shield Pro that does allow the apps directly on the device play music to all speakers so it does seem to be specific to the ATV.