Keep the highest-quality track (like DTS-HD MA or TrueHD) and perhaps one standard AC3 track for compatibility with older devices.
"Remuxing" is the process of taking the original video and audio and putting them into a new, cleaner container without losing quality.
Retain only the languages you need. Be sure to keep "forced" subtitles—the ones that only appear when characters speak a foreign language. 2. Clean Up and Remux
Look for the largest video track. If your source has multiple large tracks (common with "extended" or "theatrical" editions), you may need to check the segment map or duration to find the correct one.
To put together a "good feature" from an MKV file—especially if it is a raw rip from a disc—you typically need to clean up the file by stripping out unnecessary data and organizing the high-quality tracks you actually want to keep. 1. Identify and Select the Right Tracks