According to my Sonos Arc, the music is 2.0/PCM
Where's the 5.1 or Atmos?
Dunno.
Just checked, link still works for me unless I'm on my iMac internal speakers, then reverts to stereo.
The app needs to see a surround-capable device at the end of the HDMI cable.
Sometimes when my stuff hangs, cable reseats & power cycling wake them back up.
Are you using an Apple TV 4K?
Do other Apple music links in this thread give you Atmos streaming?