You are running into the limitation of Smart TVs and TV apps.
A soundbar by itself will not help, as no TV supports TrueHD passthrough from TV apps. TrueHD will still transcode.
A different TV would not help, as no known TV supports TrueHD audio.
If you want to direct play TrueHD audio, you need a soundbar or receiver that supports TrueHD audio and either a Nvidia Shield Pro or Xbox attached to the soundbar, not to your TV.
Shield/Xbox ← HDMI → Soundbar ← HDMI-ARC → TV
The Shield and Xbox are the only Plex clients that passthrough TrueHD audio and support 4K HDR video. Plex’s PC/Mac applications, Plex for Windows/Mac and Plex HTPC, will passthrough TrueHD audio. However, they do not (yet) support 4K HDR video.
You cannot attach the Shield/Xbox to the TV, because the KD-65AG8 does not support HDMI-eARC. It supports HDMI-ARC which cannot pass TrueHD audio. So no Sonos soundbars, as they do not have HDMI inputs.
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