Use of alsamixer and pulse audio volume control (pavucontrol) were suggested quite early in the thread (posts#2,#3) and since repeated, I guess that route to success may be insufficient.
It would maybe have helped if the OP posted screen shots of their alsamixer and pulse audio results to aid diagnosis. But heyho we are were we are.
Lately it has been Intel sound hardware that has been prominent regarding sound issues with ZorinOS/Ubuntu, so odd that AMD/ATi hardware, which is usually directly supported by linux kernels. So my thoughts were then maybe use a different kernel.
I have just done a websearch using as searchterm: "Rembrandt Radeon High Definition Audio Controller No Mic Ubuntu 24.04"
and found this: https://askubuntu.com/questions/1552769/no-audio-after-fresh-install-24-04
Maybe that will help or give some clues. It may also be worth the OP doing other websearches with that searchterm, considering Z18 is based on Ubuntu 24.04.
Supplementary Question: Does the Mic work when you run "Try Zorin" from the live USB?
EDIT: Just thought. Noone has asked about installation conditions. e.g. BIOS Secure BOOT, Fast Boot, Windows fast start-up. Maybe worth looking at pre-installation advice here to see if anything was missed: Before you install