Installing Medibuntu will solve your problem... After installing this package you will not only able to play mp3s but any media format you wish to play.(using bunshee or any other player)
The following bash command adds Medibuntu's repository to Ubuntu. It also adds Medibuntu's GPG key to your keyring, which is needed to authenticate the Medibuntu packages.
This command should be run in the Terminal (Applications → Accessories → Terminal):
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
Medibuntu's repository is deactivated by upgrading to a newer Ubuntu release, so you should run this command again after the release upgrade.
You may also wish to add the following packages. The first will cause many apps from the Medibuntu repository to appear in Ubuntu Software Center. The second will allow users to generate crash reports against Medibuntu packages and submit them to the Medibuntu bugtracker.
sudo apt-get --yes install app-install-data-medibuntu apport-hooks-medibuntu
Please note you may have to use --force-yes instead of --yes in order for this command to succeed.
Another good option.
Instead of tyrying above steps...
Just open ubuntu sofware center and install VLC player. And make it your default player. You will be able to play any file format you wish to plat.