My headphones are the only thing I ever plug into the audio out, and yet when I plug them in, Windows 10 refuses to play any audio through them until I use the popup to tell it that they are indeed headphones. This is especially annoying when I don't have the ability to interact with the popup right away (for example, when media is in full-screen mode).
It's an old standard, so I can allow that it's probably hard or impossible for the OS to tell, but I imagine there there should be a way to configure Windows to always assume it's headphones or otherwise avoid the popup and play audio, considering that the popup is a new feature in Windows 10--or at least, one I never encountered in Vista or 7.
HKEY_CURRENT_USER\Software\Realtek\Audio\RtkNGUI64\General
.. look in there.. you might be able to hack your way to what you want.. but probably not. You can also find settings underHKEY_LOCAL_MACHINE\SOFTWARE\Realtek