there is a guide available at *ubuntuguide.org
now u can install softwares from menu Applications>Add/Remove.
otherwise u can go to menu System>administration>Synaptic package manager.
u needs admin rights to install any software with apt.the guide above will help enabling mp3,wma,wmv etc enabled in Ubuntu,these plugins/support was not in many Linux distros bcoz of the License issues.
just follow the guide to enable all repositories.
btwn this needs to be in open source section.