Hdmi capable video cards have there own sound cards to do their sound rather than use your laptop's sound card. Make sure you have the drivers installed.
Here is the nvidia vid card steps as a example
To set up digital audio on your graphics card
From the NVIDIA Control Panel navigation tree pane, under Display, click Set up digital audio to open the associated page.
Click Change Windows Sound Settings.
The Microsoft Windows Sound settings panel opens, from which you can configure your system's audio output.