Convert mkv files and import them into imovie

I'm using iMovie 10. I'm not willing to upgrade to iMovie 11 because that requires os 10.15 and if I upgrade to os 10.15 some of my apps will cease to work. In any case, I have some mkv files. When I convert them into any format compatible with iMovie I run into problem. I'm converting them using Python which uses the module ffmpeg. I'm not willing to buy some 3rd party app that converts the mkv files because they cost money and ffmpeg should be able to do everything I need to do. When I convert the files to mp4 the files work fine with VLC player but after importing them to iMovie there is no video. When I convert the files into .avi or .mov the audio is missing.

The solution provided here does not work, namely, just changing the extension of the file.

https://discussions.apple.com/thread/2282019?login=true