Install ffmpeg (on a Mac):
brew install ffmpeg --with-libvorbis --with-sdl2 --with-theora
Use ffmpeg for the conversion:
ffmpeg -i audio_input_file.m4a -c:a copy video_output_file.mp4
This works very quickly because it just wraps the audio file in a video container. Would like to eventually include a relatively simple image on every frame of the video.
The OP on Reddit was trying to use this line which does include the command for including an image:
ffmpeg -loop 1 -i black.jpg -i audio.m4a -c:a copy -c:v libx264 -crf 51 -pix_fmt yuv420p -shortest output.mov