To extract the opus audio from mkv file, open a terminal window and enter the following command: $ ffmpeg -i Attention.mkv -acodec copy -vn audio.opus <enter>
To find out the audio stream in the .mkv file using ffmpeg, open a terminal window and enter the following command: $ ffmpeg -i aaa.mkv <enter>
A MKV format video file can be played back on Linux using VLC. You can get VLC from your distribution's package repository or you can download it from http://www.videolan.org/vlc. VLC is also available for Windows, and is one of the best swiss-army knife kind of media players available.