(this probably applies to various other distros as well)

Reinstalled Xubuntu 17.10 and found that VLC (VideoLAN) playback for at least 1 MKV file was resulting in audio only with no video. One possible resolution:

Go to Tools -> Preferences and enable Advanced. In the resulting tree, go to Video -> Output Modules then select OpenGL or other output. Automatic doesn’t seem to detect right, depending on configuration.

As always, YMMV