You can found the sources of libavcodec here:
 http://ffmpeg.sourceforge.net
 
You can found the sources of libvorbis here:
 http://www.xiph.org/

You can found the sources of libjpeg here:
 http://www.ijg.org/
 
You can found the sources of libdivxdecore ProjectMayo here:
 http://www.projectmayo.com/

You can found the sources of libmp3lame here:
 http://www.sulaco.org/mp3
 