That appears to be the long-since deprecated UWP app. You need to download and install the new Plex for Windows https://www.plex.tv/media-server-downloads/#plex-app
or the new Plex HTPC app Introducing Plex HTPC!
After connecting your computer per HDMI to your AVR, go into the preferences of these apps and set the connection type to HDMI.
Then activate the checkboxes of all the audio formats which your AVR can decode.
