You can use sys-usb is your audiovm. But maybe you’ll have to configure it for this.
It’s not working in any of your VMs or only on those based on the disposable template for which you’ve changed audiovm from dom0 to sys-usb?
Try to make bluetooth headset work for you using some test AppVM instead of disposables.
Maybe you need to properly configure sys-usb as audio qube or create separate sys-audio:
Or you can just attach your USB bluetooth controller to the qube where you want to use it.