Use the USB cable as an audio interface when the phone is plugged in Whenever I'm at my desk, my phone is plugged into my PC and I use both my phone's (internal) headphones and my PC's (external) headphones. The reason is that I use my phone for podcasts and my PC for everything else. 

Is there a way (an app) to make the USB cable behave as an audio interface? So my PC would see an extra microphone (which would be the phone's audio output), and I could just set my PC's headphones to reproduce whatever is coming in from the phone. This way I could get both audios delivered through the same headphone, instead of having to use two.

I realize I could use a regular audio cable with two out's, but I'm trying to reduce the amount of cables running around my desk. :)
I'm not sure if I made myself clear. So please complain if I didn't.
I don't think there is, but you could use a Bluetooth A2DP headset which would allow you to wirelessly listen to audio being outputted by your phone.

Some PCs / laptops can also output audio from a paired bluetooth device like a phone.