It is pretty impossible to get an explanation of why you will need this package, "libavcode-extra", rather than plain "libavcodec". See for instance this ubuntu forums question: http://ubuntuforums.org/showthread.php?t=1423948 (it is from 2010 and still unanswered). But my guess is that you won't have for instance mp3 ability without it (no, I didn't test that). After all the "MPEG-1 & -2 Audio Layer III" specifications are copyright by Fraunhofer IIS et al., and these companies have every right to one day ask everyone in the world to pay for the use of it... not a likely scenario, but still it could possibly be the reason for having a libavcodec that does not support it.