When connecting to Zorin OS 16 with RDP, all is fine except audio doesn't play on client

Hi, I have installed Zorin OS 16 and everything runs fine. I can select audio Optical out or HDMI just as I prefer.

Now I want to use a remote session with RDP (since I use that for other systems with Windows as well). I use Microsoft RDP. On Zorin I did:
$ sudo apt install xrdp
$ sudo systemctl enable --now xrdp
$ sudo ufw allow from any to any port 3389 proto tcp
Now I connect from MacOS with Microsoft Remote Desktop to Zorin OS 16 and it all seems to work fine. Video plays fine and response is very good. The audio device in Zorin now shows 'dummy output'. In MRD client side the setting is 'Audio play on this system'. No audio can be heard on the client side. I cannot find any setting that would be required to be enabled.
Does anyone have an idea how to fix this?

Thank you very much!
Marcel