Right-click on the speaker icon in the system tray (bottom right of screen) and select Playback Devices, you might have an option on here for playing back via HDMI.
He mentioned that he doesn't have the option in the playback tab 😉
This laptop has NVIDIA Optimus, so the HDMI isn't even being powered by the NVIDIA GPU, but instead the Intel HD Graphics. To get the audio working properly on the laptop, install the Intel HD Graphics drivers from the ASUS website for your laptop.
(You may have to install the one or Windows 8.1, as there isn't a Win10 specific version available, but it will work just as well.)
I have installed Intel HD graphics driver from ASUS website and there is no audio option whatsoever. Playback settings looks like this.