I just upgraded to Windows 8.1 yesterday, and since then I've been unable to get my sound to work over HDMI.
It's capable of making *one* sound after Windows starts up, and after that it stops working.
If I try to test the speakers in Sound configuration, it gives me a dialog that says "The device is being used by another application. Please close any devices that are playing audio to this device and then try again"
I've had this problem before in the past, but I was able to make it go away by going into the Properties for the HDMI Output and deselecting the checkbox that says, "Allow applications to take exclusive control of this device." Now even that no longer fixes the problem.
I also thought of trying to uninstall the new drivers for the AMD High Definition Audio Device, and putting back the ones I had before, but unfortunately those ones were for Windows 7 64-bit, and it seems that Windows 8.1 will no longer allow me to use Windows 7 drivers.
The video card I'm using is a Radeon HD 4550 (Getting the video drivers to work in Catalyst again was another long and painful story, but I'm past that at least).
This is a home theater PC, hooked up to surround sound by HDMI, so without this working, my PC (and projector) is basically useless. Somebody PLEASE help.