However, with great flexibility comes great complexity. How do you extract a specific audio track? How do you add subtitles without re-rendering the video? How do you repair a broken movie file?
MKVToolNix (often referred to by its core tool or "mix") is a free, open-source suite used to create, alter, and inspect Matroska (MKV) files. It is primarily a multiplexer (muxer) mkv tool mix
Do confuse MKVToolNix with video conversion. It is a remuxer (changes the container/package) not a transcoder (changes the codec). It cannot turn AVI into MP4 or shrink a 50GB file to 1GB. However, with great flexibility comes great complexity