5.1channel

Two options:

1. Use the optical S/PDIF output of your motherboard.

2. Use the HDMI output of your GPU. (This is what I'd recommend if you're using a HDMI display).

Go to control panel>sound>playback devices

For option 1, connect the cable and set the 'Realtek digital output' as you default device, then right click it, go to properties, and in the rightmost tab select DTS or Dolby Digital or 5.1 (or something like that).

For option 2, connect the HDMI cable through the receiver and set the 'Nvidia HDMI Output' as your default device. Then click 'Configure' and select 5.1.