If VLC doesn’t produce sound maybe it doesn’t have the required audio
codec. mkv is container, it can contain streams encoded with various
codecs.
The method I use for adding codec support to SLED without installing
third party applications like VLC or MPlayer or replacing all your
gstreamer packages with ones from packman is written up at ‘Additional
multimedia codec support for SLED 11 SP1 | Novell User Communities’
(http://www.novell.com/communities/node/13146/additional-multimedia-codec-support-sled-11-sp1)
Whether that will give you the necessary codecs to play these mkv files
I couldn’t say. If you can put one of the files somewhere I can download
it, I’ll try playing it.