btw, don't forget to "refresh" your uPnP AV server on the DNS-323 before trying to play the new .mp4 file. Until you do the refresh, it will still present it as .mkv to your device (TV, BD player, etc).
ok, as an edit, most .mkv files will play, but some won't, and some don't have audio, especially if the audio is a separate file...
