Plex only plays PCM 2.0

[lahouter 0]

I have a ATV 582 connected via HDMI to a Yamaha AC Receiver then to TV. I have configured the ATV 582 audio to HDMI Passthrough. When I use Kodi my tv series plays plays Dolby Digital via the amplifier. No problem. However Plex (my preference) only play PCM 2.0. All the settings in Plex have been set to allow audio output as 5.1. What settings could I be missing? Anyone having a similar issue (or even perhaps success)?