WIndows 10 vm gnome-boxes no sound?

Hi NixOS friends!

I have a small but annoying problem. On my machine I run two windows 10 VMs in qemu/kvm and they work just fine.

My problem is that I can’t get the sound to work when attaching to the machines using gnome-boxes.

The strange thing is if I connect to the machine using virt-viewer or virt-manager then the sound (within the vm) works just fine.

I am using:

virsh dumpxml win10 | grep audio
  <audio id='1' type='spice'/>

Any hints what is causing this? (Or how to troubleshoot)

B.t.w All other sound within nixos works just fine. It is only in gnome-boxes that I have this problem.

Thanks in advance,

Michael