I would like to use the button on my headphones (the one that play/pauses on a phone) as an input on my pi. I understand that this may need an expansion/audio board. Is it possible?
You most likely will need an expansion board that splits the 4 pin from the headphones to the gpio on the pi. Plus the audio coming off the pi's audioport. Depending on how well you can program for the pi you should be able to. The button on the headphone grounds the pin. And the device counts how many times its pressed in a certain amount of time.