Comment 6 for bug 363805

ok,

when you install kdenlive many dependencies are installed, one of the most important is libmlt1 (the framework kdenlive relies on) and if you look at this : http://packages.ubuntu.com/jaunty/libmlt1
you will see that libmlt1 depends on many ffmpeg libraries (libavcodec,libavformat,libavutil,libswscale) and you can see that, example for libavcodec, there are two alternatives : libavcodec52 or libavcodec-unstripped-52

by default when you install kdenlive the "simple" libs are installed not the "unstripped"versions, this is because the unstripped versions activate codecs like faac or x264 with more or less patents in some countries.

if you want to create an mp4 file with ACC sound and H264, unstripped libs of ffmpeg are mandatory. You must install them manually, like I did :
sudo apt-get install libavcodec-unstripped-52 libavformat-unstripped-52 libavutil-unstripped-49 libswscale-unstripped-0