Actually, I found going into Device Manager and disabling and then re-enabling the "Intel(R) Display Audio" driver solved the problem for me.
- Right click on the start button, and select "Device Manager."
- Look for "Sound, video, and game controllers," and expand its section.
- Right click on "Intel(R) Display Audio," and select "Disable."
- Then right click it again, this time selecting "Enable."
This was done on the anniversary update with the latest drivers. You may have to try unplugging and then plugging back in the HDMI cable between steps 3 and 4.