Thank you for posting this.
I lost count the number of hours I spent trying to fix this issue. I even resorted to removing the hotplug detect pin from one of my hdmi cables.
I know I tried some drivers older than 12.1 but they didn't work either.
Must be because, as you said, they did not actually install over the newer driver.
This time I used driver sweeper first.
I skeptically installed 12.1 and the sound FINALLY comes back after switching TV inputs!