how to install decoders to listen a mp3 songs

actionpartnership there is a typo in you command...

In short please open a Terminal from the menu Applications → Accessories → Terminal and type or copy and paste, one row a time, then press enter:

sudo apt-get update
sudo apt-get install ubuntu-restricted-extras

give your user password when requested, you don't see nothing when you type it, then press enter.

Then to get better audio and video support read below...

-----

Play mp3 and dvd under Ubuntu install skype googleearth acroread and other stuffs using Medibuntu

First please install https://help.ubuntu.com/community/RestrictedFormats

You need to have extra repositories enabled..

Please first enable the universe and multiverse repository:

Open System → Administration → Software sources → [ Tab Ubuntu software ]

enable "Community-maintained Open Source software ( universe )"
enable "Proprietary drivers for devices ( restricted )"
enable "Software restrictecd by copyright or legal issue ( multiverse )"

Close and confirm the repository reload.

Type the following command in a terminal (applications → accessories → terminal)

sudo aptitude install vlc smplayer mplayer

To get better dvd playback and optional packages here the medibuntu available software list http://packages.medibuntu.org/
you need to add the medibuntu http://www.medibuntu.org/ repository to your /etc/apt/sources.list file:
( here the medibuntu howto https://help.ubuntu.com/community/Medibuntu but see below )

In short please open a Terminal from the menu Applications → Accessories → Terminal and type or copy and paste:

sudo wget http://www.medibuntu.org/sources.list.d/`lsb_release -cs`.list --output-document=/etc/apt/sources.list.d/medibuntu.list; sudo apt-get -q update; sudo apt-get --yes -q --allow-unauthenticated install medibuntu-keyring; sudo apt-get -q update

-- give your user password when requested, you don't see nothing when you type it, then press enter

Now to install, type:

sudo apt-get update
sudo apt-get install ubuntu-restricted-extras libxine1-ffmpeg
sudo aptitude install libdvdread3 libdvdnav4 libdvdcss2 regionset gnome-mplayer
sudo aptitude install non-free-codecs w32codecs
sudo aptitude install gstreamer0.10-pitfdll gstreamer0.10-plugins-good
sudo aptitude install gstreamer0.10-plugins-ugly gstreamer0.10-plugins-ugly-multiverse

give your user password when requested, you don't see nothing when you type it, then press enter.

You can also install other Medibuntu repository provided optional software http://packages.medibuntu.org/ :

sudo apt-get install skype
sudo apt-get install googleearth
sudo apt-get install acroread

Hope this helps

Thanks actionparsnip, that solved my question.