Hi there,
You can adjust the volume of your rear speakers using either SmartThings or the provided Soundbar remote. By design, the surround speakers are supposed to simulate sounds happening behind the viewer. Therefore, most of the time the surround channels are only used for ambient sounds, which are naturally lower in volume.
You will need to make sure that you set the Soundbar to Surround, Game, or Adaptive. If you leave the Soundbar in Standard, the rear channels may not properly send to the rear speakers.
~Samsung Solutions