How can I get multi channel audio through the HDMI port on a early 2015 MacBook Pro?

I am trying to get 7.1 channel audio to run out of my MacBook Pro to a theater set up. The display works great the instant I plug the HDMI in however the audio still comes out of the macs speakers. I have checked the settings and no HDMI output is available. An airplay option is available but that only outputs two channels max to the receiver. When I go to source setup on the receiver it receives video at 1080p 60herts however it specifically says No Audio From Source.

Also I have connected my iPhone 6s Plus using an Apple adapter through the same HDMI cord and it works perfectly. We also use the HDMI with our PS4 occasionally and it works perfectly even with 3D.

So the MacBook Pro is early 2015 with an i5, the receiver is a Harmon Kardon AVR 2700 (we are running 7.1 channels out of that) and we are connecting through the front AUX HDMI port through a 50ft HDMI. The receiver is sending the audio to a projector at 1080p 60 hertz if that changes anything...

Also if it helps the Receiver supports dts-hd master audio decoding, Dolby True HD/Digital Plus decoding, HDMI 1.4(the HDMI is 2.0 compatible) , as well as multichannel input, and PCM, and Linear PCM.