I have been trying to get WDTV subtitles working with mp4 format so I don't have to keep separate format for Roku. So far I learned a few things. First is that WDTV does not appear to understand internal subtitles inside mp4 container. I was able to multiplex subtitles using ffmpeg as well as mp4box. In both cases VLC was able to play subtitles but WDTV was not. The second thing I learned is that Roku does not appear to understand multiplexed subtitles ether. This fact is apparently documented on Roku forums and elsewhere online. The third thing I learned is that WDTV is pretty bad at playing mp4 files. Somehow the playback is very choppy and the video freezes a lot. Monitoring consumed bandwidth I found that it is very low and is not enough to play the content smoothly. Converting the file in to mkv container without transcoding, fixed all playback problems. The bandwidth consumption became much more even and video played without any issues. This was a bit surprising as Roku played the same mp4 file rather well.