To merge video.mp4 with audio.mp3
static ffmpeg build for osx downloaded from
http://evermeet.cx/ffmpeg/ffmpeg-2.8.1.7z
ffmpeg -y -i video.mp4 -i audio.mp3 \
-c:v copy -c:a aac -strict experimental \
-af afade=t=in:ss=0:d=1 -af afade=t=out:st=4:d=1 \
-map 0:v:0 -map 1:a:0 -shortest output.mp4