How can I play mp3 files or videos?
Install ubuntu-restricted-extras
Also have a look at the Ubuntu wiki page on restricted formats.
Use the terminal and install restricted extras:sudo apt-get install ubuntu-restricted-extras
Or just search "Ubuntu Restricted Extras" (or just restricted") in your software center and install the ubuntu restricted (as opposed to xubuntu or kubuntu).
The codecs are not default in Ubuntu so you may need to install them following any of the above links you also could install other players such as VLC player and make it your default, totem and rythmbox also could be updated to have the missing codecs cheers