Best answer by Chris
Ambient Mode - the Rear speakers are always surrounds and when watching TV it will be dependent on how much 5.1 mix is sending to those rears if you hear nothing at all. When listening to music the rears act like old Dolby Prologic where you get a little bit of mix from the rears but not much.
Full Mode - the rear speakers when watching TV still are just surrounds so still may not play much if the mix coming to them from TV has very little surround info. HOWEVER, if you play Sonos music they act as a full stereo pair giving you full stereo sound when listening to music via controller/online source.