You should investigate it further, looks like a bug. Maybe report to issue tracker on github. Maybe you will be able to improve sound for everyone.
What do you mean when you say fullscreen? That resolution of Windows is the same as display native resolution?
Windows resolutions are limited to some pre-set list of those. If your display size is there you can make windows qube look full screen. I would use devilspe2 rule to move window to (0,0) and remove decorations (to emulate fullscreen).
If your display size is not provided in the list - it makes things harder. You will have to compile seabios to include custom resolution. Because, unfortunately, Qubes OS does not support arbitrary resolutions in Windows qubes like VirtualBox can.