HDMI Output only detects 2 channels not 5.1

I've connected my Dell XPS 13 9350 to my AVR 5.1 surround sound home theater system. Unfortunately the HDMI only outputs 2 channels and is greyed out, not allowing me to select 5.1 or 7.1. How do I update or upgrade my Realtek audio drivers to support more channels?

My issues are similar to this thread for windows vista, but no one seemed to have a fix

http://www.eightforums.com/sound-audio/6655-digital-audio-hdmi-only-has-2-channel-option-2.html?s=3d1d98220abe44571db26994a8467802

Could someone help me get 5.1 output from my HDMI source?

Thanks,