I picked up a TCL 4-series with Android TV during an early black Friday sale. I have a 5.1 system connected via toslink/optical (doesn't have HDMI input). The system supports dolby digital and DTS, but not dolby digital plus. So far, I've found that only Netflix outputs true 5.1 surround sound (i.e., converts DD+ to DD). Prime video and HBO Max both only put out stereo. This is regardless of what audio setting I use. If I set audio as "Auto" the apps send a fake 5.1 signal that is really just 2.0. If I set it as "passthrough" then it just sends a 2.0 signal. This seems to imply that only the Netflix app does DD+ to DD conversion, and the rest just revert to 2.0.
I've read that Nvidia shield, Mi Box, Fire stick, etc all do DD+ to DD conversion. I've also read that "newer" TVs usually support DD+ to DD conversion. Is there any way to enable this on the built-in Android OS?
UPDATE: I received one of the $30 Dynalink boxes and I'm getting 5.1 surround on Netflix, Prime, HBO, and NovaPlayer (with DD+ file). So the limitation does appear to be the TCL Android OS and not the apps themselves.
Although the TV has support, apps must also have support for the conversion, in that case, Prime Video still doesn't support the conversion, the same for HBO.
All you have to do is to contact each customer service and make a formal complaint about the absence of the aforementioned conversion in those apps.