Spotify ‘connect’ doesn’t support the Atmos audio format, it’s stereo only? Are you sure that Atmos audio is showing as the input in the Sonos App (check both the badge on the ‘Now Playing Screen, or the Arcs ‘Audio In’ in the ‘About My System’ device list?)

I’m not expecting atmos through spotify connect - but the sound on spotify playing through the PC is a lot thinner than spotify connect

It’s most probably ‘thinner’ because you have perhaps been using ‘Full’ audio for music playback via your Sonos App or Spotify App and so getting the stereo output on both the front and rear channels of your HT setup, but now you are playing audio over the TV/Computer connection via HDMI, are now experiencing only receiving PCM stereo through the Arc’s two front channels only - there is not a ‘Full’ setting for what I will refer to as TV playback in the Sonos software and I think that is perhaps what you’ve encountered.

Such a setting has been requested however in the past by some users here and I too would like to see that feature added for music playback over a TV connection and Sonos have acknowledged that request but they do not announce their development roadmap for a variety of reasons.

At least that’s what I think you are perhaps referring to in your posts here.