I've not tried this so can't vouch for its use - however I've looked at my copy of Mplayer's Makefile and it seems to have an uninstall option so....
As previously stated
Go to the directory where you built the source of Mplayer
run - su -c "make uninstall" and give the root password when prompted.
This should run 'make uninstall' as root.
You may have codecs lying around that you have installed and you'll have to deal with these seperately.
If you wish to make Mplayer from source I would recommend running a "Slackbuild" which will make a package for you. You can then install/uninstall with pkgtools (installpkg/removepkg).
There is a build on Slackbuilds.org :
http://slackbuilds.org/repository/12...media/MPlayer/
If youre unfamiliar with building a Slackbuild have a look on the site for instructions - its really quite simple and very convenient.
Running a basic "./configure,make,make install" can leave files all over the place - sometimes with no way of uninstalling them ! You are always better building a package using one of the tools or perhaps building a package yourself from scratch.
Good luck..