Which has control of the outputs, Cubase or windows?

I have noticed that Windows 8.1 works differently to Windows 7 in that on a clean boot, if Cubase is the first thing which is loaded; then it has control of the outputs but if an app such a Windows Metro player is used before cubase is started, it does not relinquish control.

Is there anything in the driver model that can force an application to release the driver, since I note unlike Windows 7, Windows 8.1 actually knows when another application, i.e. Cubase is using a particular output, e.g. if those outputs are also assigned for Windows sounds.

Now before I am jumped on for using my outputs for email bleeps and alerts, it worked under Windows 7, albeit with the odd hiccup and now MMCSS is more robust I thought we could go the extra mile in this regard.