Ok, now that I understand what the problem is, and why you want this. ssh into your N2 and use the following command, to mute the audio on the hdmi.
amixer sset 'Audio hdmi-out mute' on
If this has the desired effect for you I can add it as a setting in the gui.
edit: just so you are not questioning your memory. It’s the older devices(s905/s912) that has the option in the gui to mute the hdmi audio. This was probably removed for the newer devices because the way it was done on the older devices doesn’t work for the newer devices(s922x/s905x2/a311d).