MKVToolNix is the essential toolkit for working with Matroska (MKV) video files.

Merging Files

Open MKVToolNix GUI. Drag your video file, audio tracks, and subtitle files into the “Input files” area. Each track appears in the “Tracks, chapters and tags” list. Check/uncheck tracks to include or exclude. Set language and track name for each. Click “Start multiplexing” at the bottom to create the MKV.

Extracting Tracks

Need just the audio from an MKV? Add the file, uncheck everything except the audio track you want. Under “Output” tab, change the file extension (e.g., .mka for audio-only MKV). Multiplex. You now have a standalone audio file.

Splitting by Duration or Chapters

Output → Splitting → “After output duration.” Enter a time like 00:30:00 to split the video into 30-minute segments. Or choose “After specific chapters” and enter 3,6,9 to split at those chapter markers.

Editing Headers

File → Header Editor. Drag an MKV file. Edit track names, languages, default/forced flags, and aspect ratio without remuxing. Fix files where the language is listed as “und” (undefined) by setting the correct language code.

Adding Chapters

Chapter Editor → New Chapter File. Add chapter entries with timestamps and names. Save as XML. Then in the Multiplexer, add the chapter file in the “Output” tab under “Chapters.”