What setting would that be because on the manual I never saw anything like that. I'll look and see after work.
There may not be one. Stereo is left/right. So soundbar plays left/right. Rear speakers are rear left and right. Stereo signal does not have that, so soundbar does not use the rear speakers.
Let's say you force them to play, which rear speaker would be playing the left audio, is it left based on the speaker as you are facing it, or as you are facing the front speakers? So you can have left audio on left front, then left audio coming from behind you from the right as a possibility. Then you have a mess with imaging. Leave music playback to the front speakers and the sub.
More speakers does not equal better sound. The only really good way a bunch of speakers may sound OK is if you play audio in mono, so all the speakers play the exact same thing, but then you want speakers that are similar so the audio is balanced.