Ubuntu – Failed to download repository information due to missing CDROM

Seems to me like a network issue, are you behind a proxy? If so, your configuration might be borked.

Have a look at your etc/apt/apt.conf file and check the proxy settings there.

(Added by user in question after discussing issue on comments)

SOLUTION
miguel@miguel-VirtualBox:~$ cat gedit /etc/apt/apt.conf
#Wrong!!!
#Acquire::http::Proxy "http://proxyconf/proxy.pac";
Acquire::http::Proxy "http://17X.XX.XX.X:8080";

It was an issue with the network settings - an invalid proxy was setup causing the problem.

It sounds like you've run dpkg --add-architecture 1386 at some point. If dpkg --print-foreign-architectures shows 1386, running dpkg --remove-architecture 1386 should fix the problem.