Hi,
do you have by chance an ATI Radeon video card? I am having the same problem, and I read that in Batocera the same issue happens. The solution (in Batocera) appears to be: "HDMI Audio is disabled by default due to a black screen issue on some monitors. To enable it, edit on the USB key both boot/grub/grub.cfg and EFI/boot/grub.cfg files by adding radeon.audio=1 at the end of the linux line".
However, in Recalbox did not work for me (or I did something wrong).
The information I have just pasted is from:
https://wiki.batocera.org/doku.php?id=supported_hardware
Anyone has found a solution for Recalbox PC (v6.1.1) ?
Regards,
Gabriel