Currently I am converting MP3 to MP4 black video using following command line in Windows Registry:
C:\Program Files\ffmpeg\bin\ffmpeg.exe -loop 1 -i "C:\Program Files (x86)\Utilities\BlackVideo\black640x480.png" -i "%1" -c:v libx264 -tune stillimage -shortest "%1.mp4"
It works fine, but FFMPEG isn't that smart to just encode a chunk from keyframe to keyframe and then repeat it without encoding. Instead, every second of the video is encoded, which is slow and causes unnecessary CPU load.
To solve this problem I created a black video with no sound for the first 10 seconds. Now I'm looking for the right FFMPEG command line options to loop this video without re-encoding for as long as the MP3 track lasts.