@Strit is compiling the new kernel 4.19.76 for the RPi 4 and should be in the repo for upgrading in a few hours.
This new kernel will have an enhancement that will show each HDMI's sound as devices and will allow you to choose which HDMI' sound you want to use via the Volume Control's Mixer Settings on the taskbar. You Set which HDMI's device you want to use by clicking/un-clicking it's "Set As Default" button under the Output Devices tab.
Using HDMI 0 for sound:
Using HDMI 1 for sound: