After a few false starts, I got it working. Installation and configuration were surprisingly easy, once I found a procedure that aligned with Debian. I was impressed with the quality and flexibility of the virtual machine manager user interface. Sounds from the nested guest operating systems were passed all the way through to the speaker.I will try it on a minimum server installation of Debian 12 first to see how it works. I am not concerned about sound.
In case anyone cares, I installed QEMU / KVM in a Debian 12 virtual machine. The VMware virtual machine that served as a host was a fresh Debian 12 installation with "hypervisor applications" enabled in its VMware settings. Once the QEMU/KVM software was installed and running, I created another Debian 12 instance as a guest inside that virtual machine. It ran perfectly, and there was sound output through the nested virtual machines. (I heard beeps and boops from the operating system UI in the guest, but did not go further.)
Where the initial installation attempts went wrong on the Debian 12 host:
I tried following the procedure in the video link from above, but ran into issues around figuring out "Debian apt" equivalents to the Pacman packages in Arch. After a couple aborted attempts, it was clear that I was off the path and headed for the weeds.
What worked for a Debian 12 host:
I used a web search to find this simple procedure that worked for my Debian 12:
I created a Debian 12 virtual machine guest using the GUI manager interface. If you have seen and used other virtual machine software, the essential options should look familiar. I was able to create a new Debian 12 guest using the .iso of the Debian 12 net installer. The guest booted the installer, connected to the internet to download the rest of the installation files, and it just worked. The guest ran very slowly, but I was pushing against RAM limitations in my host Mac to run a guest that acted as the host for its own guest.
In case it matters, the 4 real + 4 virtual core Mac has 16 Gbytes RAM. The original Debian 12 guest that served as the QEMU host was allocated 4 cores, 12 Gbytes RAM, 60 Gbytes drive. The Debian 12 nested QEMU/KVM guest was allocated 2 cores, 2 Gbytes RAM, and 20 Gbytes drive.
This work helped me make progress on a small research project I started, related to Linux installers.