This change may break some things but it might be helpful for some.

1. it copies all audio - e.g. if mkv has 2 audio tracks - it copies them both

2. itdetects subtitles in the source. if source is mkv and you are on Linux and you got mkvextract - it unpacks
them and uses them

The result of above changes: if you have a goot mkv file with audio and subtitle tracks - they are copied.