Cant get 5.1 output on movies

Hello to everybody. I have a problem with my setup. I own a Sony 55XD8599 android tv and a Sony HT-RT3 home theater. I have installed the Kodi 16.1 and I have chosen the 5.1 audio output at the advanced settings of kodi.
The problem is that I only get the front bar from my speakers to work and not the rear ones. I have also tried both HDMI and optical connection. I have contact Sony but they dont know what they are doing. You are my last help.
If anyone knows what settings to change on the whole set up, I would appreciate it.