This thread was really helpful for me to set up an audio qube. I followed some of its steps since I reinstalled QubesOS when R4.1 was realeased and lost the setup described here.
The difference is that I wanted to use Bluetooth and had sys-usb handling it. So in my case I decided to make sys-usb my audiovm – there may be some vulnerabilities associated with that but I was too lazy for searching how I could attach the Bluetooth device with sys-audio, as it was not appearing in the USB drop-down menu. So in the above steps, instead of creating a new VM, all I had done was to perform the steps in the sys-usb VM. Bluetooth headset is working properly and also the keyboard shortcuts described above.
One more hint, I was missing a systray icon from which I could control volume. As I don’t know how to place the xfce4-pulseaudio-plugin for the sys-audio (sys-usb in my case) in the main xfce panel (which runs in dom0), my partial solution was to add a launcher widget to the panel and set its command to qvm-run --pass-io sys-usb "pavucontrol". So now I don’t need to open a terminal in sys-audio and type pavucontrol to open the mixer. There might be a better solution to this, but that’s what I have for now.
PS: I actually took some additional steps to install the required packages to run audio and usb services in a fedora-36-minimal fresh template. From this template I made a disposable template in which the modprobe file was added. The autostart scripts were placed in sys-usb.