You need to add the following lines to /etc/apt/sources.list file or you need to make sure you have enabled Universe and multiverse repositories in /etc/apt/sources.list file

sudo gedit /etc/apt/sources.list

Make sure you have the following two lines save and exit your file

deb http://archive.ubuntu.com/ubuntu hardy universe multiverse
deb-src http://archive.ubuntu.com/ubuntu hardy universe multiverse

Now you need to run the following command to update the source list

sudo apt-get update

Install mplayer using the following command

sudo apt-get install mplayer

if you want to open mplayer just type

gmplayer

or

You can Open from Applications—>Sound&Video—> Mplayer Movie Player

Install libdvdcss2 and w32 video codecs in Ubuntu 8.04 (Hardy Heron)

Support for WMV, RealMedia and other formats has been bundled into the w32codecs package. This package is not available from the Ubuntu repositories due to licensing and legal restrictions.

For Ubuntu 8.04 (Hardy Heron) Users run the following command

sudo wget http://www.medibuntu.org/sources.list.d/hardy.list -O /etc/apt/sources.list.d/medibuntu.list

Then, add the GPG Key using the following commands

sudo apt-get update

sudo apt-get install medibuntu-keyring

sudo apt-get update

For i386 Users install Codecs using the following command

sudo apt-get install w32codecs libdvdcss2

For amd64 Users install Codecs using the following command

sudo apt-get install w64codecs libdvdcss2

Using above download locations you can install most of the mutimedia codecs for ubuntu.

Mplayer Plugin for Firefox

If you want to install Mplayer with plug-in for Mozilla Firefox run the following command

sudo apt-get install mozilla-mplayer