If you are cueing up your next track on one Deck and the currently playing track from the other Deck is audible on your headphones as well, the MIX knob for the headphone output in TRAKTOR may not be set to 0%. The MIX knob controls how much of the Main output signal is added to your Monitor output (headphones). Make sure the MIX knob is turned fully counter-clockwise if you want to listen to the cued track only.