I have been effected by this for over 6 months now. After I upgraded to a A10-7850K from my old A8-6600K I started to experience very loud audio crackles when using HDMI. Switching back to the A8-6600K or using analogue fixes the audio.
Here is a git hub ticket with more information and multiple combinations of the newer APU's and GPU's experiencing the issue. Loud audio crackles and static with HDMI · Issue #144 · ValveSoftware/SteamOS · GitHub