By default; Windows always sets the speaker configuration to 2 speakers (or Stereo), so in order to setup a 5.1 speaker system you will need to go through the Control Panel.

Start-->Control Panel-->Hardware and Sound-->Sound-->Configure-->choose your configuration

Just follow the steps, click " Apply " , then Okay.

You may also have to set the audio in the BIOS to HDMI

Hope this helps

B.