8. Add an empty sound in an audio stream

for sequence:
ffmpeg -f lavfi -i aevalsrc=0 -i 30fps/blend4web%*.png -r 30 -c:v libx264 -preset veryslow -t 9 -map 0:0 -map 1:0 -shortest -y intro.mkv

for video:
ffmpeg -f lavfi -i aevalsrc=0 -i input.mp4 -vcodec copy -acodec aac -map 0:0 -map 1:0 -shortest -strict experimental -y out.mp4