Plex Media Player, PMP, is the only Windows/Mac Plex client that can pass multi-channel audio. Edit: There is also the Plex for Kodi add-on and PlexKodiConnect.
Plex Web and Plex for Windows/Mac (the new client) do not have that capability.
PMP is staying around to address the needs of HTPC users. Details in the Plex Blog, https://www.plex.tv/blog/desktop-af/.
To access the audio options in PMP, first enter TV Mode by selecting the “TV” box in the upper right portion of the PMP window. Then click on your username and select Settings.
Additional details:
