This has been mentioned a couple of times. Incus includes a package of Qemu which means that when you install Incus, you also get an installation of Qemu that is separate from your system’s Qemu installation (if you installed one anyway).
Therefore, Incus has a nice and fresh Qemu for the Incus VM instances. A recent change in Qemu 8 requires to append --audio spice to the existing line. See also, How to run a Windows virtual machine on Incus on Linux – Mi blog lah!