Or maybe you have an old video card around?
I suspect that the AV capability of my workstation is provided by my RTX 3070 and there is no alternative AV source from my combination of Gigabyte B550 Vision D-P motherboard and AMD Ryzen 7 5800X processor. If true, that would explain why my USB to HDMI adapter is not working.
Your other suggestion of using a spare video card might do the job, though won't older cards fail to handle newer audio codecs?