You have to set the Quantum as the default audio device for your computer.
PC: Right click on the speaker icon in the Taskbar. Set the Quantum's Main L/R outputs as the Default Device.
MAC: Open the Audio MIDI Setup from the Utilities folder. Set the Quantum as the Sound Output Device.
Keep in mind that Quantum output channels are discreet and separate.
Mains = 1/2
Line Outs = 3/4, etc...