Ok, I made some test on Intrepid.
Installing ubuntu-restricted-extra fixes the bug because it replaces libavcodec51 with libavcodec-unstripped-51.
libavcodec51 is a dependence of ffmpeg that installs it by default (as other packages).
But ffmpeg dependence is libavcodec51 OR libavcodec-unstripped-51, so we should install this last by default to do the trick.
Ok, I made some test on Intrepid.
Installing ubuntu- restricted- extra fixes the bug because it replaces libavcodec51 with libavcodec- unstripped- 51.
libavcodec51 is a dependence of ffmpeg that installs it by default (as other packages).
But ffmpeg dependence is libavcodec51 OR libavcodec- unstripped- 51, so we should install this last by default to do the trick.