Routing question

In Cubase I have set up the control room. My monitors are routed as UR44Mix1L and R. I have headphones set up as UR44Mix2L and R. This works fine, but how can I hear non Cubase sound through the headphones for example a reference track from my WMP or Youtube? That audio is streamed through the UR44 L1 and R1 and goes through the Mix 1 output on the hardware. Many thanks, Mike.