If you have an HDMI output, it should show in the Playback Devices on the sound Panel. If you do not show anything, you may need a driver for that device.
You may need the HDMI cable plugged in to see the options. You are not sending sound anywhere else? You system may be able to deliver different sound from the front or back ports, but I don't think it can support two sound sources at the same time from the back.