Interesting! At work I hacked together a video writer that compiles still frames to a movie using OpenCV. But it is slow --ungodly slow. I'll have to check out ffmpeg!
ffmpeg -i frame%04d.png -vcodec mjpeg -sameq test.avi ffmpeg -i myvideo.mkv -vn -acodec ogg myvid_sans_video.ogg ffmpeg -i myvideo.mkv myaudio.mp3