Hi @Dukdlx 

Welcome to the Sonos Community!

This is expected behaviour, or at least, it is depending on some settings.

When you play a music source via the Sonos app, your Arc is aware it’s music and plays to both itself and to the rear speakers (either at the same volume as the Arc, or at lower level, depending on the room’s settings in the Sonos app).

When you play from a TV source, if the source is in stereo, then the Arc will attempt to up-mix and play some of the sound to the surrounds. If, however, the source device is playing stereo music but delivering it to the Arc as a surround sound format, then as far as the Arc is concerned, the rear channels are supposed to be quiet and they will remain so.

My recommendation is to play music via the Sonos app - that way, your TV doesn’t need to be on in order to play, and you have more control over what the rear speakers do. Otherwise, you’d need to adjust settings on your Apple TV which would probably result in no surround sound when watching movies and TV, without changing the settings back again.

Note that the TV’s settings could also alter this behaviour. If, for example, the TV’s audio output format is set to Dolby Digital, then the Arc would always receive Dolby Digital, regardless of the format of the source, resulting in the same behaviour as in the previous paragraph. Auto, Bitstream or Passthrough would be better options here.

I hope this helps.