Change default audio track/subtitles on mkv files with mkvtoolnix

First thing, have a look at the properties of the mkv file that you’re going to edit.
You can use mkvinfo but i prefer Mplay, it’s more clear.
Just open the file and with a right-click you can see the list of the audio tracks and subtitles.

Choose the default audio track that you want and take note of the track number (let’s image you want the 3rd one).
Now set the default flag for that track to 3 and the others to 0:
mkvpropedit filename --edit track:a1 --set flag-default=0 --edit track:a2 --set flag-default=0 --edit track:a3 --set flag-default=1

Do the same for the subtitles (set everything to 0 to disable subtitles):
mkvpropedit filename --edit track:s1 --set flag-default=0 --edit track:s2 --set flag-default=0