Your Motherboard has one HDMI output and regular HD Audio plug-in ports no DP Port output.
The DP Audio must be coming from your GPU Card and not the Motherboard. Which means it is using the Nvidia's or AMD's GPU Driver HD Audio and not the motherboard's HD Audio drivers.
Also the GPU card can have both HDMI and DP outputs on it. When you mentioned that HDMI works fine is that from your speakers being connected to the Motherboard's HDMI port or your GPU card HDMI port?
If the DP is the one having issues than it is the GPU HD Audio driver being used.
I would be make sure you have the latest Motherboard CHIPSET installed and the latest BIOS version compatible with your processor.
Also check Device Manager for any errors that may cause your issue with the DP Port.
Use Windows own Audio Troubleshooter to see if it sees anything wrong in Window's Settings - Troubleshoot.
Also make sure Windows if fully updated via Windows Upate.
I would check the settings on whatever Speakers or Audio equipment you are using on the DP Port and on the GPU Settings menu.