It seems that getting output via hdmi on this laptop is "mission impossible".
The only reference of an (unsuccessful) attempt is here:
and it's pretty old.
I tried various newer alsa versions and nvidia drivers version with no success.
I'm currently running kernel 18.104.22.168 (with alsa 1.0.20, previously tried 1.0.21) and the 190.42 nvidia driver but still aplay -L doesn't show the hdmi audio output.