You can disable the audio from your BIOS configuration; however, this means that that there will be no audio from the audio jack, HDMI or the mini-DP.
I have tested the behavior you mentioned, but the issue did no happen. I played a YouTube video, then plugged an iPhone earphones and they sounded just fine.
Try the following;
- Enter your BIOS setting, and press F9 to load defaults.
- Install the latest Audio driver, see here for it; Download Realtek ALC* Audio Driver for Intel® NUC Kits.
- Make sure that in your Playback Devices you select your headphones.
Maybe reinstalling Windows 8.1 is the best option since the WLAN driver causes BSOD and bluetooth device is error 10. I download and install every driver with Intel Driver Update Utility but don't know why the system is unstable now.