No audio while playing some mkv files

I am trying to play an mkv file.  Half of my HD movies that are mkv files will not play.  Some of them however, play fine if I change the audio setting.  Any suggestions?  I did convert one movie from mkv to mp4.  It took nearly 20 hours!!!  Now I cant seem to get it to transfer over to my external HD because of an error message there,  PLEASE HELP!

use the optical connection and your problem is gone

So. just use the optical connection and run it to my surround sound?

Worked Perfectly!!!