First I used this manual.
So we're in luck it has RCA jacks that are outputs. But most if not all of these do not downconvert to stereo from HDMI when the audio source is other than Stereo.
See if you can find settings in those apps to send only stereo.
Even the Geek Squad was thrown on this one.
I have a RCA input/output soundbar(acoustabar) from 2010. It worked perfectly fine with my 1080p tv, but today I updated to a 4K (VizioD65u).
The red and white RCA cables connected from the TV to soundbar result in no sound, however when I run a HDMI cable to the cable box/appletv and run the RCA audio cables from them into the stereo, it works!
The audio on Netflix and HBOgo do not work through the soundbar. It results in loud static anytime I try to play an episode, but works fine when just navigating through the menu screen of both apps.
Apps like Hulu, Trailers, ESPN, and ShowtimeAnytime work just fine.