Setting up headphones/speakers as an Input Recording Device.
 

Like if I wanted my headphones/speakers to act as a microphone, so I could have something playing on my headphones/speakers, and if I am talking to someone online, they could hear it?

Doubleclick on the volume icon rightbottom of the screen, go to File > Options (or settings... I have a dutch install), find your way to the record tab. That can be done either by changing the dropdown at the top, or selecting "record" as a radiobutton. When you found it make sure the "sterio-mix" selectbox is selected.

Now click OK and you will have a new volume panel, now select Sterio-mix as your default record stuffy.. Now all recording streams will have the same data that goes to your output devices such as the boxes.

You prob won't find it in one time though, just play around with settings etc. It differs on all computers with different soundcards.