How are the AppleTVs connected to the network? Wi-fi would be a stretch for UHD without transcoding. With the AppleTV, it's already having to transcode the audio somewhere, likely on your Plex server machine. If you're streaming Wi-fi, it's also then probably down-converting the video to something it can push through your wi-fi.
When you ripped the disc, did you select everything on it, or did you pick and choose which titles to grab? Some discs have "spoof" information in them, so it's possible you ripped the wrong title. I can't remember exactly what I grabbed from that one, I've ripped so many in the past month.
The thing that made me really dig in and check what was happening with the audio is that I was trying to watch Pirate Radio the other night, which is only 1080p, and it was stuttering. I went to my server and found the Transcode settings on minimal effort, which was not enough to convert the DTS audio track to DD+ on the fly while ripping a disc, apparently. I bumped up those settings dramatically to take advantage of the horsepower in my server, since all it's doing is feeding me movies, and I didn't have any issues for the rest of the movie, but that's when I started researching why it was having to transcode anything at all.
