Try to set the SPDIF as the default audio device. That might work.
If it doesn't, try updating audio driver.
At last, as you said, your GPU driver is the last one, still, the problem might be right there. Usually, when the device manager shows the HDMI audio as functional and it still doesn't work, it's probably a GPU driver issue.
Try switching your GPU driver to older but stable versions.
On my case, nVidia drivers area always a pain in the *ss, so, when I successfully find a good and stable driver I stick with it for a long long time.