S2419H, Apple Mac, built-in monitor speakers not working

Hi,

I have just bought a pair of S2419H monitors and connected to a Mac via HDMI. They are recognised and the video works, but the built-in speakers do not produce any sound. Am I missing something? Do I need an audio cable? I thought HDMI would do that? Or are there updates / steps I need to do? Can't see anything on the websites and have followed the suggestions and fixes but still not working. I have two screens and neither work, neither individually nor together.

Thanks