Force audio only transcoding, with direct play video
I'm running Plex Server on QNAP and streaming MKV files to Roku 3.
I would need to disable video transcoding completely but still have the Plex to transcode the audio track (AC3) to something my system can handle (such as AAC).
If I convert the audio stream to AAC with MkvTools or such, the direct play with Roku 3 works fine, but i have to convert all files beforehand. If I don't convert the file beforehand the "direct play" only plays video without audio, because my system can't digest the AC3. On the other hand, the automatic transcoding tries to also transcode video which is too intensive for my QNAP's CPU. I think it could handle audio only transcoding, though, if that was somehow possible to set on Plex. I don't know why it even tries to transcode video since it plays fine without transcoding (tried looking at the log files, but didn't come up with anything..).
So, i guess the bottom line is:
Is there a way to force audio-only transcoding for MKV containers with AC3 audio stream without touching the video stream at all?