Today, after the usual

apt-get update; apt-get upgrade

procedure on my Debian box, I noticed that FFMPEG wasn't working as expected anymore. It happend that the upgrade installed a new version of FFMPEG from the official Debian repositories, while I had used the deb-multimedia.org repository for this package before. The official Debian package lacks support for libfaac. However, I had to switch back to the old package. Here is how I've done this.

APT allows to set priorities for certain repositories and packages. Debian calls this feature "APT pinning". All I had to do was to set the priority for the FFMPEG related packages from deb-multimedia.org to values >1000 to make apt always install FFMPEG from there. To do this, just create a new file in /etc/apt/preferences.d. I called that file deb-multimedia. Here are the contents of the new file:

Package: ffmpeg
Pin: origin www.debian-multimedia.org
Pin-Priority: 1001

Basically it says: "Use the www.debian-multimedia.org repository for the ffmpeg package no matter what.". Debian plans to replace ffmpeg with libav in future releases, so I additionally made APT never ever install libav by creating another file in /etc/apt/preferences.d:

Package: libav-tools
Pin: origin ""
Pin-Priority: -1

Pin-Priority: -1 tells APT to never install that package. After these few steps it is possible to switch to the old repository by simple reinstalling the packages. Great work Debian, thank you for this!