Open your sources.list:
sudo gedit /etc/apt/sources.list

Add this line to the bottom:
deb http://medibuntu.sos-sts.com/repo/ feisty free non-free

Run these commands:
wget -q http://packages.medibuntu.org/medibuntu-key.gpg -O- | sudo apt-key add -
sudo apt-get update
sudo apt-get upgrade

ffmpeg should automatically upgrade.