Note the QEMU since has been ready since 4.0, in April 2019 so 2 years.
We dropped the ball on getting this implemented in libvirt, since we
had almost no config options for sound at all in libvirt. We had just
hardcoded 3 sound backends based on the graphics frontend.
So in terms of historic libvirt compatibility, we've only ever relied
on the QEMU_AUDIODRIVER env, none of the other million audio env vars.
IOW, if QEMU was to be conservative, you can drop all env vars except
the main QEMU_AUDIODRIVER.