I run the following command for converting an M4A audio file to an MKV video file, using a single image for the entire video duration:

ffpb -loop 1 -framerate 1 -i "image.png" -i "audio.m4a" -c:v libx264 -preset medium -crf 0 -c:a copy -shortest "video.mkv"

FFPB will show a progress bar for a phase, but for the second phase it will not show.