After installing 12.04, the headphone plug no longer mutes my external speakers.
-Sound settings - Analog Output - Analog Stereo Duplex Mode - Test Sound
Left Front sounds thru Left external speaker
Right Front sounds thru Right Headset
This sounds like something you can configure using the alsamixer.
Open up a terminal, and simply type in "alsamixer". On the right side of the various volume controls, you should see an "automute mode". Simply tap the up arrow key on your keyboard to enable it, and then press escape.
If there is no "automute mode" on the mixer, then you can manually mute the speakers when you have your headphones plugged in, by simply highlighting the channel that has the speakers attached and pressing the "m" key.
For future reference, please post the specific hardware you are using when you're having a problem. It makes diagnosing things much easier, because we can look for known bugs.