A few weeks ago AMD began releasing technical documentation
on the HDA audio portion of their Radeon GPUs. Yesterday AMD published the second version of their HDA programming documentation.
Version 2 of the AMD HDA Verbs documentation adds in information covering HBR compressed audio pass-through and audio/video delay verbs (lipsync). This updated HDA documentation adds in eight more pages to the original ten page document.
The HBR compressed audio pass-through documentation notes, "The HBR Control allows for the transmission of compressed audio using the maximum audio channels in the HDMI/DP link (eight) instead of the regular two channels used for compressed audio."
The updated AMD HDA audio GPU documentation can be found in this PDF file