I have 30 mkv files that I want to change the default audio/subtitle-track on. Doing this in MkvToolNix would take a great deal of time. So I was thinking about writing a script. But for that I need to know how to use mkvmerge to do this.

Current structure:
ID 1: Video
ID 2: Audio 1 Default
ID 3: Audio 2
ID 4: Subtitle 1 Default
ID 5: Subtitle 2

I want track 3 and 5 to be default tracks instead!