Ok, I found a solution to this issue that works, at least for me. It seems that the installer package for the HD 4400 Graphics drivers is failing to install the "Display Audio" driver. Without the Intel "Display Audio" driver, there can be no audio over HDMI.
This means that the Display Audio driver must be loaded "manually" using the Device Manager in Windows. The process for diagnosing this problem and manually adding the Display Audio driver is clearly described by BoulderJ over at Windows Eight Forums. Here's the link...
BoulderJ's post is near the bottom of the page.