I recently bought a 5.1 channel Pyle system that supports Dolby Digital and Dolby Digital Plus. I can get full sound from external equipment, local channels and even Netflix. However, no other app - even YouTube will produce 5.1 audio, for the videos that are encoded with Dolby Digital, which work on my desktop hooked up to the tv.
My TV is a Skyworth Android TV 8 based tv.
Maybe those are outputting 5.1 DTS and your audio system doesn't support DTS?