Problem with synaptic.

In the terminal, type "sudo apt-get install ubuntu-restricted-extras"
For more information about that, check this link: https://help.ubuntu.com/community/RestrictedFormats

did exactly of what you said but didn't help me;
this is what it shows me when i type the command in terminal

Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package Ubuntu-restricted-extras

also tried installing amarok but it shows this
Amarok cannot be installed on your computer type (i386). Either the application requires special hardware features or the vendor decided to not support your computer type.

am just wondering wether my cd is corrupted or what ?

Probably not, but it doesn't seem like the computer is looking in the repositories. Was your computer connected to the internet when you did this?

If yes, could you copy the result of the command:
cat /etc/apt/sources.list

This might help us understand why it won't install the package necessary to make the MP3 playback work.

deb cdrom:[Ubuntu 7.10 _Gutsy Gibbon_ - Release i386 (20071017)]/ gutsy main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.

# Line commented out by installer because it failed to verify:
#deb http://in.archive.ubuntu.com/ubuntu/ gutsy main restricted
# Line commented out by installer because it failed to verify:
#deb-src http://in.archive.ubuntu.com/ubuntu/ gutsy main restricted

## Major bug fix updates produced after the final release of the
## distribution.
# Line commented out by installer because it failed to verify:
#deb http://in.archive.ubuntu.com/ubuntu/ gutsy-updates main restricted
# Line commented out by installer because it failed to verify:
#deb-src http://in.archive.ubuntu.com/ubuntu/ gutsy-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## universe WILL NOT receive any review or updates from the Ubuntu security
## team.
# Line commented out by installer because it failed to verify:
#deb http://in.archive.ubuntu.com/ubuntu/ gutsy universe
# Line commented out by installer because it failed to verify:
#deb-src http://in.archive.ubuntu.com/ubuntu/ gutsy universe
# Line commented out by installer because it failed to verify:
#deb http://in.archive.ubuntu.com/ubuntu/ gutsy-updates universe
# Line commented out by installer because it failed to verify:
#deb-src http://in.archive.ubuntu.com/ubuntu/ gutsy-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
# Line commented out by installer because it failed to verify:
#deb http://in.archive.ubuntu.com/ubuntu/ gutsy multiverse
# Line commented out by installer because it failed to verify:
#deb-src http://in.archive.ubuntu.com/ubuntu/ gutsy multiverse
# Line commented out by installer because it failed to verify:
#deb http://in.archive.ubuntu.com/ubuntu/ gutsy-updates multiverse
# Line commented out by installer because it failed to verify:
#deb-src http://in.archive.ubuntu.com/ubuntu/ gutsy-updates multiverse

## Uncomment the following two lines to add software from the 'backports'
## repository.
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
# deb http://in.archive.ubuntu.com/ubuntu/ gutsy-backports main restricted universe multiverse
# deb-src http://in.archive.ubuntu.com/ubuntu/ gutsy-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository. This software is not part of Ubuntu, but is
## offered by Canonical and the respective vendors as a service to Ubuntu
## users.
# deb http://archive.canonical.com/ubuntu gutsy partner
# deb-src http://archive.canonical.com/ubuntu gutsy partner

# Line commented out by installer because it failed to verify:
#deb http://security.ubuntu.com/ubuntu gutsy-security main restricted
# Line commented out by installer because it failed to verify:
#deb-src http://security.ubuntu.com/ubuntu gutsy-security main restricted
# Line commented out by installer because it failed to verify:
#deb http://security.ubuntu.com/ubuntu gutsy-security universe
# Line commented out by installer because it failed to verify:
#deb-src http://security.ubuntu.com/ubuntu gutsy-security universe
# Line commented out by installer because it failed to verify:
#deb http://security.ubuntu.com/ubuntu gutsy-security multiverse
# Line commented out by installer because it failed to verify:

Ok, so it would appear that during your installation of the system, you didn't have access to the internet so it commented out the repositories you need.

Bhavani Shankar's got the answer right there. If your computer has access to the internet, it should solve the issue right away.

If it doesn't, you can re-enable the repositories directly by doing "sudo gedit /etc/apt/sources.list", and in the resulting window, remove the # before each line that starts with "deb". Then, save, exit and in the terminal, type "sudo apt-get update". You should then be able to install MP3 playback with "sudo apt-get install ubuntu-restricted-extras"

Thanks Bhavani Shankar, that solved my question.