Changing the input frame rate of a series of images in FFmpeg
With image sequences, you have to use the framerate option. So,

ffmpeg -framerate 30 -i %d.png -itsoffset 0.770 -i soundtrack.wav -vcodec mpeg4 -y "ffmpegtest.mp4"


With -r specified before the sequence name, FFmpeg assumes 25 fps and duplicates/drop frames to meet the set rate.