Here are some tips for anyone having trouble getting HDMI audio to work with nvidia cards, particularly those like me who had it working under Sierra but had it break under High Sierra.
Start by downloading HDMIAudio.kext 1.1 from here
https://www.dropbox.com/s/9xenemmfwa1ee7b/HDMIAudio-1.1.dmg?dl=0
You'll also need ioRegistryExplorer from here
https://mac.softpedia.com/get/System-Utilities/IORegistryExplorer.shtml
Fire up ioRegistryExplorer. In the search field, look for HDAU. That should narrow things down to just your audio card. Take a look at the properties on the right.. You'll see vendor-id and device-id. Vendor-id should be <de 10 00 00> for any nvidia card. Your device-id will be <xx xx 00 00>, where xxxx is your ID. Mine was <ba 0f 00 00>, so BA0F is my device-id. Yours is probably different. Make a note of it.
Mount your EFI partition, and put HDMIAudio.kext in EFI/CLOVER/kexts/Other.
Next, fire up clover configurator and load up your config.plist. In the "kernel and kext patches" section, create a new entry under KextsToPatch. Use com.apple.driver.AppleHDAController as the kext name. Put DE101A0E in the "find" column, and DE10XXXX in the "replace" column, replacing XXXX with your device ID. I put DE10BA0F there.
Reboot, and cross your fingers.
This patches AppleHDAController on the fly, replacing one of the valid IDs (DE101A0E) with your card's ID. That was enough for me to get HDMIAudio.kext to work as well as it did in 10.12 Sierra.
Not sure if this works under Mojave, I'm waiting for new web drivers to be released before I upgrade further. However I don't believe that this kext edit would cause any problems if done properly under any macOS version.
Good luck!