.mkv is a video file and stand-alone FFMpeg will convert one to stand-alone .mp4. Another video format. I believe Audacity with FFMpeg installed will open up the audio portion of an .mp4.
There may be a shortcut there somewhere, but that should work. I’m looking.
So i made the test with your file and it worked flawlessly however i tested an mp4 file and got the same result as for mkv, i don’t know to edit video so i just want to use audacity to touch a bit the audio behind the image.
Are you able to play the mkv file in VLC?
If it plays correctly, you can use VLC to convert to WAV format (audio only) which can then be imported into Audacity (See: https://www.vlchelp.com/convert-audio-format/)