Here are my "relevant" system specs:
CPU: 5960X
Motherboard: ASUS Rampage V Extreme

This seemed to have worked fine under Windows 7, but it stopped working awhile ago, I "think" about the time I upgraded to Windows 10. My Line-in will not playback any audio through my headset, I'm looking at my Recording Devices and I notice that the microphone on my headset is receiving sound when I talk into it (the volume bars increase/decrease depending on how loud I talk), but after plugging my monitor into my Line-in and also trying my phone, I cannot seem to get more than 1 bar of sound. I tried two different cables, same results. I know for a fact that Realtek HD is detecting when I plug something into the port, as SupremeFX appears and wants me to confirm which port I plugged something into.

Can someone help me on the issue?