Hi, HDMI audio output is experimental and not enabled by default. I believe it’s still rather flaky and isn’t guaranteed to work, but it has worked for me and my LG monitor. You can enable it by editing /etc/default/grub
Change the line for GRUB_CMDLINE_LINUX_DEFAULT to include “apple_dcp.hdmi_audio=1”. The line should look something like this (yours may be different):
GRUB_CMDLINE_LINUX_DEFAULT="quiet rootflags=subvol=root apple_dcp.show_notch=1 apple_dcp.hdmi_audio=1"
Once you have finished editing, I’m pretty sure you need to run sudo dracut --regenerate-all --force then restart to put the new settings into effect.