Hi so I have a Audio 2 DJ set-up with a Macbook (Garageband), MIDI controller, plugged into a sound system.

Right now I'm just trying to get the sound to come out of both outputs, but can't seem to figure out how. When I play notes on the keyboard in Garageband, sound comes out of output A through the system but not the headphones that I have plugged in to output B. How would I be able to hear myself in the headphones?