I never added medibuntu repositories to my default ones (the default "Ubuntu" repositories) and I never needed to add 'anything' except installing VLC and restricted extras to be able to play DVDs.

Did you follow the suggestions in this thread? Just install these and try playing again -
Code:
sudo apt-get install ubuntu-restricted-extras libdvdread4
If you still can't play DVDs, run the player (I'd recommend VLC) from terminal and post back the messages it leaves in the terminal.