HandBrake is a free, open-source transcoder we can use to convert MKV to MP4, and indeed works across most video formats. It works on Windows and Mac alike.

For example, you could have a device that can only play MP4 videos, and not MKV. To be able to watch your MKV file, you have to know how to convert MKV to MP4 file format.

How to convert MKV to MP4 without losing audio or subtitle tracks, degrading visual quality, or waiting too long for the final MP4.

I'm going crazy trying to find a way to convert a .mkv file with H264/AAC to an .mp4 file with the same content (so QT can play it natively). VLC's transcoding doesn't work, ffmpeg doesn't seem to ...