Known issue, from 2014.
Old conversations here already had given indications.
Try a search… The solution was double: first, WD TV Live has a feature that goes to check on the Internet for album covers atc, and the file can be considered “music” and not video, so you will have only the audio.
Other problem is that the latest firmware does not plays some .mp4 video files, and you should rollback to previous firmware.
In old conversations there are also instructions on how to do that.
Hope you can resolve
here is the link
