The new Lattice USB 3.0 Video Bridge Development Kit simplifies integration of USB 3.0 audio and High-Definition (HD) video connectivity for a wide range of applications, leveraging the ...

Importantly, the USB Audio Device Class 3.0 continues to support both analog and digital audio. The two secondary bus (SBU) pins are still allowed to be used for non-digital audio transfer, so ...

They are taking digital audio and converting it to analog for the headset. In effect the USB speakers have a built in sound card. Also, I notice you have the RealTek digital output set as default.