Comment by xp84

Handbrake is fine if you truly need to reencode (aka “transcode”) your video, but if you find yourself with a video that your player can’t read, you might be able to just change the container format (remux it) using ffmpeg, copying the video and audio streams directly across.

With video there are 3 formats: the video stream itself, the audio stream itself, and the container (only the container is knowable from the extension). Formats could technically be combined in any combination.

The video stream especially is costly in CPU to encode, and can degrade quality significantly to transcode so it’s just a shame to re-encode if the original codec is usable.

Container format mkv is notorious for not being supported out of the box on lots of consumer devices, even if they might have codecs for the audio and video streams they typically contain. (It has cool features geeks like, though, but for some reason it gets less support.)