I think ripps has been packaging mplayer2 for some time now and some way along the road renamed the package to reflect this. Just install mplayer2 instead.
sudo apt-get install mplayer
The following packages have unmet dependencies:
mplayer : Depends: mplayer2 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
This is a good example of where apt-get falls short while aptitude might have done the trick. When dependency errors show up aptitude will try to offer solutions (eg. "don't install mplayer, install mplayer2 instead"), often several such, whereas apt-get merely concludes it's an unresolvable dependency and exits with errors. On the other hand, apt-get seems to be better at keeping track of automatically installed packages and tags them for removal when they're no longer needed (when using autoremove).
So when you get errors like this, you may want to try the same command but with aptitude. Add the argument -P to force it to always ask for confirmation before proceeding. Do note that apt-get autoremove won't work well on removing automatically installed packages it didn't install itself, so extensively mixing the use of the two is not a good idea over time.
The aptitude in 12.04 is broken on multiarch platforms (like amd64) and basically only works when installing packages, else I normally recommend the use of aptitude for everything besides updating package lists. It still works fine in 11.10 though, so feel free.
As always, huge thanks to you for packaging this, ripps181.