I've tried running plex server off both the NAS and my mac mini with no change in result. Seems that some (but not all) AAC / h264 files will play video, but no audio when using the xbox one app. Appears that plex is doing direct play of the media.
Those files work fine if I play from the media server on the NAS instead of Plex. They also work fine if I use the web player in plex.
Any ideas?