Make sure that you disable the onboard sound in your Bios. This will keep windows update from re-downloading the drivers for your onboard sound.
It might of been just a glitch in your Bios Firmware. Did you try to update your Bios? The B450 boards had multiple Bios updates and 1 that makes the 3rd gen Ryzen chips work with a B450. The latest Bios update for that board is Version 2202 and it was just updated 2020/07/17. 1 of the many "fixes" included in the update is "Fixed HDMI and DP sound issue".
They had 16 Bios updates for that paticular board since it was released.
https://www.asus.com/us/Motherboards/TUF-B450-PLUS-GAMING/HelpDesk_BIOS/
Never EVER use any Realtek sound drivers that are not on your motherboards support site. Realtek drivers have always been a pain in the ass because once you install 1, it will rite over and replace the old driver but it leaves all of the old crap in your registry. Only way that I have found to fix that problem is to either spend many many hours sorting through the registry and deleting all entries and files related to Realtek (most of the time you will not get all of it) or Format and reinstall windows.
With Realtek drivers my rule of thumb is "If it works DO NOT UPDATE IT!" lol