When the HDMI's audio is not working on Windows, it's usually because the operating system isn't selecting the correct audio device when you plug in the HDMI cable. To fix that, press Win + R to open a Run dialog window, type mmsys.CPL into the text box, then hit the Enter key. In the list of audio devices in the "Playback" tab, right-click the HDMI audio device and select "Set as default device."