I purchased a steam machine type of rig with an RX 470 and was disappointed to see that the audio support is not mainlined yet. So I compiled the amd-staging-4.7 branch (as at 841c4eb9bf3cb24cecc50ba76da51f58e76c1a7a). Audio over HDMI works now - so if someone else needs this I uploaded the .debs here:
Tested on Kubuntu 16.10. Cheers!
Tested on Kubuntu 16.10. Cheers!