First, add the Medibuntu repository

sudo wget --output-document=/etc/apt/sources.list.d/medibuntu.list http://www.medibuntu.org/sources.list.d/$(lsb_release -cs).list && sudo apt-get --quiet update && sudo apt-get --yes --quiet --allow-unauthenticated install medibuntu-keyring && sudo apt-get --quiet update

Then, get gnome-mplayer and the non-free-codecs

sudo apt-get install non-free-codecs && sudo apt-get install gnome-mplayer

Restart Firefox or any other browser; or simply copy and paste the mms: url into gnome-mplayer