To do this you just need to:

- boot the Mac by pressing ALT - choose Recovery Volume
- from Utilites menu choose Terminal
- type "csrutil disable" without quotes
- reboot to normal mode
- download KextBeast and place it on the desktop: http://fs3.softfamous.com/downloads/tname-040117cb0f191/software/KextBeastv2.0.2.zip
- download AppleHDA.kext and place it on the desktop: https://www.tonymacx86.com/attachments/applehda-kext-zip.322801/
- run KextBeast and install to System/Library/Extensions
- download HDMIAudio 1.1: https://www.dropbox.com/s/9xenemmfwa1ee7b/HDMIAudio-1.1.dmg?dl=0
- open the DMG and unpack to desktop by drag and drop
- open terminal and type "sudo su" without quotes then password
- type "csrutil stat" it should reply with "System Integrity Protection status: disabled."
- inside terminal type "cd Desktop" without quotes
- then type "./install.sh" without quotes and wait for it to finish
- reboot and connect your DisplayPort/HDMI device - you should have sound and it should be visible in Audio devices