This post is to help me, because i forget the things very easy, so for me and maybe for you this is very easy.
Open applications and go to Ubuntu software centre search box write mkv
How to extract the files from mkv?
All of my files go to my Downloads folder so open Terminal and use de command cd " change directory " exp: cd Downloads
mkvextract tracks myfile.mkv 3:myfile.S_TEXT/UTF8
mkvextract tracks myfile.mkv 1:myfile.V_MPEG4/ISO/AVC
This help me to remember maybe help you to.