HDMI and Displayport are used to transmit not only graphics but also sound signals.

Do you happen to have a TV with a box from upc, Swisscom, etc.? Is the sound also transmitted via a separate cable?

Ah, thanks Gschwinds.
Then it works if I go from my ASUS Prime Z590 motherboard Intel UHD759 DP to DP on the monitor?
I didn't know that DP also transmits sound 😀💪