Have you done Trueplay to tune the speakers? I upgraded from the Playbar to the Arc during the Christmas holidays. While I do not think the Arc sounds as good as the Playbar, it has added a lot of dimension to me home theater. My surrounds and Sub are louder with the Arc. 

I have voice enhancement on, which I also did with the Playbar, for better dialogue. But even though I only have Ones as surrounds, I cannot complain about the surround levels, which are set to -2 for TV audio.

My Arc is also wired to mt network and the Sub and surrounds connect directly to the Arc’s WIFI.

First, check the Now Playing screen for the audio type to see if you are gettings Dolby DIgital 5.1/Atmos.  If you aren’t, you need to configure your TV and/or source to give Dolby.

Definitely make sure the sound is showing Dolby Atmos or 5.1 - but also, surrounds speakers will only play sound when there is something in the audio track for those particular speakers. The rest of the time, the surrounds will be quiet or have only very slight atmospheric noise. They do not play the same sound as the Arc.

I set-up Trueplay. We’re about ready to get a new tv, so perhaps I’ll wait and work on it then. It doesn't not show Dolby--just HDMI. 

You need to look at the app on your phone to see what audio format is playing. HDMI is just the physical connection. 

HDMI is the only thing I see. 

Follow-up. I updated the app, and now I see PCM 2.0 on a movie I’m watching. I saw Dolby on a few songs I streamed through apple (though not all). 

Your Sonos system can only play the format sent to it. Many movies are only broadcast in stereo (PCM 2.0). 
But, if you’re still only getting stereo on a 5.1 signal you need to change your tv settings.