By default, Ubuntu (for that mater many Linux distributions) cannot play MP3 audio files. This is because of copyright laws (makes it a restricted format) which prevents MP3 functionality from being bundled with Linux distributions.
This does not mean you cannot play MP3 audio files on Ubuntu, it just means you need to download an encoder first.
You can use Totem, which is the default media player in Ubuntu, and add the codec that will allow playing of MP3 audio files.
The easiest way to get started, is to first download or copy an MP3 file to your desktop and double click on it to open Totem. You will immediately see the following message:
Click on the Search button to allow your system to find the required software.
Next you will see the following results from the search, prompting you to make a selection.
Select the first option that include codecs for playing mp3, sid, mpeg1, mpeg2 formats, and carefully read the pop-up notice about installing restricted software. Click Confirm button to confirm the notice. Then click Install to start the installation.
If prompted, enter you password at the prompt to perform administrative tasks.
Once installation has completed, click the Close button.
The MP3 audio file that you clicked on earlier will start playing.
Next time you click on an MP3 file, Totem will open and will immediately start playing tunes for you!