The error you are getting is because you are trying to add the Medibuntu repository for 'Feisty', which was removed in October of last year. See http://packages.medibuntu.org/
You need to add the 'Jaunty' repository, which you can do by using the following command ....
sudo wget http://www.medibuntu.org/sources.list.d/jaunty.list --output-document=/etc/apt/sources.list.d/medibuntu.list
... you will then also need to add the repositories GPG key, using the commands....
sudo apt-get update && sudo apt-get install medibuntu-keyring && sudo apt-get update
You can then install the w64codecs using.....
sudo apt-get install w64codecs
As far as I'm aware, you should only need to install the w64codecs.
Medibuntu has a page in the Ubuntu Community Documentation
website, which gives you all the information you'll need, including making you aware of any potential legal issues that may arise (due to copyrights, patent issues, etc). I'd suggest reading through it, and bookmarking it for future reference.
Hope that helps