Have you tried a different headset?
Use the RealTek driver
from the motherboard vendor, if you are not using this. Sometimes there can be problems if a driver, eventhough in this case it's RealTek, is not from the PC or motherboard vendor it may not work correctly.
But, considering it's a new build, and if it never worked, it could be the ports on the case, the wiring from the front panel to the front panel headphone connection on the motherboard, incorrect connection (I know you said it's connected properly but double or triplecheck) or even a hardware problem on the new motherboard.
I built a new PC and I had problems with the front panel USB ports on the new case. It turned out the front panel USB port assembly was defective (the case vendor sent me a new one).