mkvextract.exe is a command-line tool that is part of MKVToolNix. mkvextract v8.3.0 can be downloaded from here. It allows to extract subtitles, audio, and other type of tracks from .mkv files.

Syntax is pretty simple:

mkvextract.exe tracks “D:\video.mkv” 1:”D:\track-1.aac”

Where:
D:\vide.mkv – Source file
1 – Track number ID
D:\track-1.aac – Destination file

mkvextract manual can be found here.

If you prefer to work with graphical interface, you can download a GUI add-on from here: https://sourceforge.net/projects/gmkvextractgui/