HOWTO: Install and use the latest FFmpeg and x264

[mc4man]

?

Now I think I have two sets of libavcodecs, one statically built with ffmpeg following this guide and then there is another installed with restricted extras and Medibuntu before. But I have to make sure when I go home later. One of my questions was basically I am not sure which one will be used by what...

No - you only have 1 set of the 'libavcodes', those are the shared libs that you've installed from medibuntu, they end in .so (shared object file'

Sources that were built against their 'corresponding' -dev packages will then depend on (use), those shared libs. So that would be all the repo/ppa apps/plugins that depend on the ffmpeg libs'.
To see run this something like this, using the exact name of the repo libavcodec package, on 11.10 it's llbavcodec53 or libavformat53, ect.

apt-cache rdepends libavcodec53

So if you build a source that uses the repo/ppa libavcodec-dev, libavformat-dev, ect. they will also then depend on (use) the shared libs, libavcodec53, libavformat53 ect. That was the case with k3b, we built against the repo libavcodec-dev, libavformat-dev, so it now depends on those shared libs.

When you build a source statically or against a static depend then it will not depend on the associated shared libs. A clear example is this how-to, ffmpeg is a static build, using a static x264

So the ffmpeg binary does not depend on any of the ffmpeg shared libs, nor on libx264. It does depend on all of the .so's associated with the -dev packages you installed or had installed prior to building ffmpeg

You can see this clearly here if run when your static, 'ala this how-to' ffmpeg is installed - there will be no libx264* or libavcodec53, ect. listed

ldd /usr/local/bin/ffmpeg

If you were to uninstall your ffmpeg built, install the repo ffmpeg, & run that command again you'll see the difference.

As far as the apps that use the ffmpeg binary itself. They will use whichever one that is installed. (try not to have more than 1 ffmpeg installed at once
If it's your built one then most of the time things will be fine, sometimes maybe not, they need the usually older repo version.

Hopefully this is somewhat informing, don't consider definitive or even absolutely technically correct, I mainly know whatever I think I know from observing