I have a Dell Latitude D600, and when my headphones are plugged into the headphone jack, the audio comes out of both the headphones AND the laptop’s speakers. The volume level is split in half between the two as well, 50% for the headphones and 50% for the laptop’s speakers. I would really like to know how to get all of the audio to come out of my headphones. Thank you so much.

Sounds like a hardware issue. Make sure you’re plugging the headphones all the way into the socket (sounds silly, but I’ve had that happen before).
If it’s software problem (like a windows setting or something) it wouldn’t make any sense for the volume to be split 50-50, so I don’t reckon that’d be it.
try going into the volume control system down the bottom of the screan and change the settings around, try that!