Do you have the international unlocked version? If so, this is used for the exclusive HTC app, "HTC Portable Soda". Open that little door, put a 50c coin in there, choose the desired soda on the screen using your fingers, confirm, and it will pour the selected drink via the headphone jack which is located on the very top of the device.
Originally Posted by lzicc
Please note that you have only 10 drinks preloaded in the device - after that you'll need to order a special cable on the HTC website to refill your phone and keep enjoying some delicious soft drinks on the go!