Hi Gianluca,
QEMU_AUDIO_DRV=none is most probably the problem. Libvirt is supposed to set "QEMU_AUDIO_DRV" to "spice" for VMs with graphics. Please make sure that the "Optimize for" attribute is set to "Desktop" (in Edit VM dialog) and try to shutdown and start the VM again. If the QEMU process will still have QEMU_AUDIO_DRV=none set, you can try to set the variable in /etc/sysconfig/libvirtd by adding line "QEMU_AUDIO_DRV=spice" and restart the libvirtd service.
Jakub