open a terminal, copy and paste:

sudo -E wget --output-document=/etc/apt/sources.list.d/medibuntu.list
http://www.medibuntu.org/sources.list.d/$(lsb_release -cs).list && sudo
apt-get --quiet update && sudo apt-get --yes --quiet
--allow-unauthenticated install medibuntu-keyring && sudo apt-get
--quiet update

Now,

sudo apt-get install ubuntu-restricted-extras

sudo reboot