Since Ubuntu is not a rolling-release distribution, programs like Mplayer do not receive any major update, beside bug-fixes, until each new release of the distribution itself.
Here it is a guide on how to keep Mplayer updated on Ubuntu.
First you need to add the dedicated Mplayer repository; open a terminal and run;
sudo add-apt-repository ppa:motumedia/mplayer-daily
The update the package manager’s database:
sudo apt-get update
Than you if you have Mplayer already installed, you can get the update by:
sudo apt-get upgrade
Or, if you don’t have Mplayer installed yet:
sudo apt-get install mplayer