Sound under linux and KDE seems to able to get itself confused with its settings easily enough so their is no point adding to the confusion. I’m sure that HDMI is hugely useful to those that want it and use it but up to now I never have.
Among the issues I’ve noticed is that at seemingly random times the KDE mixer Kmix will revert to setting the Nvidia HDMI output as the master channel. Then I have to change it back which starts to get old fairly rapidly.

The best way I’ve found to deal with this is by blacklisting the kernel module concerned with the HDMI audio.
With a Nvidia Card the module is snd_hda_codec_hdmi

So to blacklist snd_hda_codec_hdmi create a file called byebyehdmi.conf in /etc/modprobe.d/ (you can call it any thing you want so long as it ends .conf, and you need to be root to create and edit it)

In the file goes

blacklist snd_hda_codec_hdmi

Save and reboot

For me that does it on Arch but if it doesn’t work you may have to make sure the module isn’t loaded by initramfs at boot. On arch that would involve adding the file you created to FILES=”” in /etc/mkinitcpio.conf so

FILES="/etc/modprobe.d/byebyehdmi.conf"

(if there’s already an entry in FILES just add yours at the end, in the quotes but a space delimiting). Then run

mkinitcpio -p linux

On other distributions it may vary but the gist of it should be same.