Não tenho certeza se você está fazendo o caminho certo, mas o pacote existe:
apt-cache search restricted-extras
kubuntu-restricted-addons - Commonly used restricted packages for Kubuntu
kubuntu-restricted-extras - Commonly used restricted packages for Kubuntu
lubuntu-restricted-addons - Commonly used restricted packages for Lubuntu
lubuntu-restricted-extras - Commonly used restricted packages for Lubuntu
ubuntu-restricted-addons - Commonly used restricted packages for Ubuntu
ubuntu-restricted-extras - Commonly used restricted packages for Ubuntu
xubuntu-restricted-addons - Commonly used restricted packages for Xubuntu
xubuntu-restricted-extras - Commonly used restricted packages for Xubuntu
Você pode abrir o terminal e digitar sudo apt-get update && sudo apt-get install lubuntu-restricted-extras
digite sua senha e pronto.