Use (specifically the "Chapter Editor" or "Multiplexer" features) to split the video based on timecodes without re-encoding the video quality.
Here are the most common ways to handle an .mkv video file, typically using tools that handle high-quality straight rips without re-encoding (like MakeMKV ), or editing software:
If you need only the audio or specific subtitle tracks, MKVToolNix is the industry standard for demuxing (separating) the tracks from the .mkv container. Open your software (MKVToolNix or HandBrake).