I have disabled the EDID check using the xorg.conf option
Option "UseEDID" "FALSE"
This didn't help. HDMI audio still doesn't work. However I can now boot the AppleTV when the TV is turned off. There is more information on disabling EDID and forcing the nvidia driver to use a specific mode here:
Originally Posted by toor
I think the only solution for now is to try custom EDID. Even that 100.14.19 driver works, if I power up AppleTV with TV turned off, I have no HDMI audio. That's because the driver can't read EDID. Many people complained that nvidia driver sends fake HDMI audio even when no audio available over HDMI, so TV uses HDMI audio input instead of analog. So I think in some release NVidia has added some check to decide whether to mute HDMI audio.