Do you have ever created an mkv (perhaps with the great tool makemkv) and after playing it, you discovered that the subtitle is set on by default?

With the nice tool mkvpropedit you can simply change this. You can find this tool in the great MKVToolnix package which is avaiable for linux, Mac OsX and even Windows.

mkvpropedit file.mkv --edit track:s1 --set flag-default=0