HDMI Audio doesn't work on AMD RX580 Mac Pro 5,1
HDMI Audio doesn't work in Mojave even with supported Hardware.  My box is a MacPro 5,1 with an Nvidia GTX680.  HDMI audio worked flawlessly under High Sierra. Not at all under Mojave.  

I am part of the Beta test group and complained of this loss 6 months PRIOR to the Mojave release.  Still nothing as of today 11/30/18.