Started out in the kernel bugzilla as I figured this was an alsa driver problem:
> AMD HDMI audio output. Works fine until I suspend the machine. After resume, center channel is silent and LFE channel is sent as center.
> Massive issue as the center channel is the most used one of all in multichannel audio sources. :/
> Unloading just snd_hda_intel and reloading it seems to work around the issue.
The verdict there though was that it didn't seem like the audio end was doing anything wrong, so it must be something in the graphics driver.
AFAIK, video driver doesn't have anything to do with the mappings. Does forcing a modeset after resume help? E.g., change the mode with xrandr and see if that helps. Maybe some of the AMD specific verbs in the hda driver need to be reset?
Thanks. Going through the vendor verbs revealed the issue. Multichannel mode was getting reset on suspend, and was not properly reinitialised by the driver on resume.
Will prepare a patch for the ALSA folks. I guess you can close this bug entry as invalid.