Try selecting No sounds in Windows sound…or even better completely disable onboard audio in the bios.
Is this the hybrid version…are you using it via f/w??
If yes have you installed legacy f/w driver for windows?
Do you have a t.i f/w chipset?
You should probably also update Cubase if you really are still on 7.0.0
If you can’t solve this and you want to get working try installing asio4all and running it on that instead of the MOTU drivers.