Set HDMI to Default
You must go into your computer's audio mixer settings/properties and set the HDMI (or the HDMI device it sees) as the DEFAULT output device. This forces all sound to go through the HDMI port.
You can access sound settings by either going into the Control Panel and clicking on sound, or clicking or right-clicking on the speaker icon down in the task tray.
When you are not using HDMI for sound, you must go back and set the computer speakers as the default sound source. Otherwise you won't get the normal sound from the computer speakers.
You may also want to find out what brand the audio card on your computer is and download the latest driver from their website. It will be a little different from the standard Microsoft Windows sound driver and have more customizable settings.