Make sure you clearly label your source and destination in the Finder before you start, so that you're not yanking any tracks out of, nor sewing them into, the wrong files. The MKV file from which you want to extract the item in question.

The MKV file to which you want to add your audio track (or subtitle track, etc.).

You'll need at least two files to start with: (Make sure you first click the 'Signature' link next to the download link to get and validate the installer's checksum for extra protection against viruses.) 2. You can get either the command line tools or the GUI version via Homebrew in the Terminal, but if you understand half or fewer of the words I just wrote, just grab the regular Mac installer instead. You'll find instructions for downloading MKVToolNix at the project's site. This MKV-friendly open-source video player represents the fastest, easiest way to check your work once you're finished. But if you're allergic to UNIX commands, the Mac-friendly GUI edition provides more than enough abilities to get most jobs done.

This free set of MKV editing tools includes powerful command line versions installed and run via Terminal. Obviously, you'll need one or more MKV files to work with, along with any outside audio, video, or subtitle files you might want.What do I need to edit an MKV file's contents on a Mac?

6 best free mkv video metadata editor