I have downloaded a move (format .mp4) which has .srt sub title file. i have this movie copied to WD My book live. I am able to play movie through media share from WD TV Live but subtitles are not displaying. When i am playing the same movie from laptop, i am able to see subtitles.
You simply add a share on your NAS (probably already on by default) and connect from WDTV via shares rather than DLNA. Then you will see subtitles, both external and embedded.