Updated 07/3/2018. This solution works in High Sierra 10.13.6 as well as several previous versions of MacOS. I didn't create the tools I only figured out how to use them. Credit for the kext goes to andrejkw at macrumors. Your mileage may vary but this is the easiest solution I've found. I've used it for years & its worked for many others. Definitely worth a try.
1. Download package located here
https://www.dropbox.com/s/8hay1m0rec93vvo/HDMIAudio-1.0.dmg?dl=1
2. Mount HDMIAudio-1.0 disk image
3. Mount your EFI partition. You may need to use
Clover Configurator or similar tool to do this.
3. Copy HDMIAudio.kext from the HDMIAudio disk image to EFI/Clover/Kexts/
Other
4. Reboot. Clover may hang on the initial boot attempt, if so a restart of the computer should have you up and running.
There you have it, a simple file copy.
After the reboot I had working HDMI Audio.
*If you don't see any audio devices after reboot. Download and run AudioCloverALC command by toleda
https://github.com/toleda/audio_CloverALC/raw/master/audio_cloverALC-130.command.zip
This may help to finish off the install.
If you're confused, there is a quick rundown in this
video. *
VIDEO IS OLD. COPY KEXT TO "OTHER" FOLDER!
*Don't forget to select the proper output! System Preferences/Sound/output or hold the Option key while clicking volume icon.
So now you have HDMI audio, but what about volume control? OSX doesn't support volume control of HDMI devices natively. Use one of the utilities below to enable it.
Audio Hijack
Sound Siphon