for %%f in (*.m4a) do ( ffmpeg -loop 1 -i bear.jpg -i "%%f" -c:a aac -c:v libx264 -crf 0 -preset veryfast -shortest "video-%%f.mp4" )
for %%f in (*.m4a) do ( ffmpeg -loop 1 -i bear.jpg -i "%%f" -c:a aac -c:v libx264 -crf 0 -preset veryfast -shortest "video-%%f.mp4" )