Standard optical digital connections (SPDIF) only support uncompressed stereo (2.0) audio. They lack the bandwidth for uncompressed 5.1 or 7.1 surround sound. To get surround sound over optical, you need paid compression software like Dolby Digital Live. Analog motherboard jacks natively support true surround sound without lag.
In many common scenarios, switching to the digital output will result in :
Ensure the optical cable is pushed firmly into both ports until it clicks. You should see a red light glowing from the end of the cable before plugging it into your receiver.
If your headphones buzz when your graphics card is under load (like during gaming), switching to an optical digital cable completely eliminates this electrical noise.