Check out the following post and let me know if it helped.
http://community.wd.com/t5/WD-TV-Live-Streaming/video-mkv-wont-play/m-p/665173#M27450
Check out the following post and let me know if it helped.
http://community.wd.com/t5/WD-TV-Live-Streaming/video-mkv-wont-play/m-p/665173#M27450