I apologize as these are simple suggestions and may be something you logically have already done, but I have no idea the experience level of the OP asking questions. Have you ever at any time had sound working through your monitors speakers either with this card or another? If not you might try this monitor on another computer to verify the speaker are indeed working. Or you could plug it into a dvd player or game console to verify.
From the computer side verify that your default audio settings are choosing that HDMI audio. Much of the time your computer will have 2 or more types of audio output as you will have at least one built in the motherboard and the other available through the GPU. So in your case if the motherboard audio is the default output it won't play through your HDMI to your monitor. I found this youtube video to show how to do this if you are unfamiliar:
How to change your default speakers in Windows 10 - YouTube
If neither of these end up being the issue I would then for sure re-install the AMD driver choosing CLEAN INSTALL.
This may set things correctly again if they were not. Verify after doing that again that your HDMI audio is default for output.
Reply back for additional help if none of this helps. Or you can also request help from AMD support here:
Online Service Request | AMD
Good luck!
