I've been wracking my brain trying to figure this out. I tried making sense of the numerous HDMI audio guides here and found them non functional or too hard to follow.
I stumbled across this promising looking old link.
http://forums.macrumors.com/showthread.php?t=1499797
I followed the directions, re-booted and..... Nothing, still no audio.
After a bit of thought about the previous guide I decided to try the following on a whim.
1. Download package located here
http://patrovic.com/HDMIAudio-1.0.dmg
2. Mount HDMIAudiooo-1.0 disk image
3. Mount your EFI partition, you may need to use Clover Configurator to do this.
3. Copy HDMIAudio.kext from the HDMIAudio disk image to EFI/Clover/Kext/10.10
4. Reboot
There you have it, a simple file copy.
After the reboot I had working HDMI Audio.
This may be common knowledge but I hope this helps someone else.