Typical Debian 7 /etc/apt/sources.list
sudo vi /etc/apt/sources.list
update/add
deb http://cdn.debian.net/debian/ wheezy main
deb-src http://cdn.debian.net/debian/ wheezy main
deb http://security.debian.org/ wheezy/updates main
deb-src http://security.debian.org/ wheezy/updates main
# wheezy-updates, previously known as ‘volatile’
deb http://cdn.debian.net/debian/ wheezy-updates main
deb-src http://cdn.debian.net/debian/ wheezy-updates main
deb http://download.virtualbox.org/virtualbox/debian wheezy contrib
deb-src http://download.virtualbox.org/virtualbox/debian wheezy contrib
deb http://http.debian.net/debian/ wheezy-backports main contrib
deb-src http://http.debian.net/debian/ wheezy-backports main contrib