If you want to change default audio in dual audio permanently just use MKVToolnix.
For .mkv type file.
you can download it from here http://www.videohelp.com/tools/MKVtoolnix.
1. Open the program (I am using version 6.7.0).
2. Add the anime that you want to change.
3. As you can see there is two audio, click one of them.
4. Look at Default track flag and change them.
5. If yes,it will be your default audio(it will use that audio directly when open the file)
6. Click Start muxing.
7. Enjoy.