I did what I need to do, so I'll post these instructions in case someone finds this thread on Google. However, I have a new problem.
1) In the Realtek HD Audio Manager, in the 'Device Advanced Settings' window, tick the second button 'Make front and rear output devices playback two different audio streams simultaneously'.
2) In the 'Line in' tab, mute playback and turn recording volume to full.
3) Set the speakers as the default playback device. This will default all computer audio (foobar, firefox, etc) to play through the speakers. Setting the headphones as the default device sends media through the headphones. I'll explain my problem with this in a second.
4) In the control panel 'Sound' window, 'Recording' tab, open properties for the line in and ticked 'Listen to this device'. You can then set which output it will play through (I used my headphones, in this case).
Now I can play media through the speakers and the line in through my headset. However, I now realize I'd much rather have media audio sent to both. I need to figure out how to play computer audio (firefox, etc) through both the speakers and the headset. Any ideas?
Here's a picture so this actually makes sense.