# cut the video to match the time of the audio
ffmpeg -i input.mp4 -ss 0 -t
# replace the audio
ffmpeg -i input.mp4 -i input.mp3 -map 0:0 -map 1 -vcodec copy -acodec copy output.mp4
# adding an overlay to the video, replace the 0:0 with x:y coords
ffmpeg -y -i input.mp4 -i image.png -filter_complex 'overlay=0:0' -strict -2 output.mp4