Headphones Don't Override Speakers
 

Okay, so I'm playing music and the sound is coming out of the speakers.

When I plug my headphones into the front of my computer, sound comes out of my headphones, but the speakers still make sound, also.

I'd like the speakers to stop producing sound when the headphones are plugged in, so only the headphones are making sound. Is there a way to set things up like this?

AFAIK, that should be a hardware feature. If your soundcard supports detection (Intel 865 boards and onwards) of inputs of devices at ports, it should be able to mute the speaker automatically, on plugging in the headphone. Software, typically, won't allow you to do anything particularly. If your volume control settings show options for setting volume for speakers and headphones independently, you can mute the speaker volume.