There are quite a few different factors, including if the OS supports HDR (macOS and Linux currently don’t), and mpv (the player engine behind Plex Media Player) passing the HDR metadata through to the OS. There are options (such as the NVIDIA Shield) that allow passthrough of HD audio and support HDR, and then others (like the Apple TV 4K), which support 4K HDR media, but do not pass through the audio.
