My media works fine on all other devices (a couple smart TVs, web, tablets, roku 2 LT) but I've run into a problem recently whereby older files I have (and definitely worked fine when I first ripped them) play back faster on the Roku 3 box. I had previously fixed this by forcing the Roku box output to PCM/Stereo, and all seemed okay (originally the videos had faster visual playback, and also chipmunk-stretched-audio). However, after the most recent server update I no longer seem to be able to do this. Setting the Roku to PCM stereo output does fix the chipmonk effect, but the video still plays back much too faster, and constantly buffers.
I investigated a bit to see if I could determine why (the files worked fine on the Roku 3 when I watched them last year). I found a video file that works fine, and one that doesn't and compared what was different. The only difference I found was that I'd encoded one with 'aac (LC)', and the other more recent one with 'aac (HE-AAC)'. The Roku seems to play the latter incorrectly. I reencoded the audio to AAC LC and all was fine.
One thing I have noticed is that various tools I tried to load the mkv into incorrectly reported the HE-AAC samplerate as 24000 (handbrake, mkvtoolnix, etc.). Wondering if Plex is making the same mistake and so telling the roku box to play back faster? The videos were transcoding, so potentially the transcoder reading/encoding the videos incorrectly?