Hello,
1) Do you know the use of the Cue-> Master button?
It is described on the video here
The Cue to Master rotary is made to select whether what you hear in the headphones is
- the preview (the deck(s) you select with 2 push buttons with headphones over the deck volume fader)
- or the Master, if you have activated the Master button below the Master volume vumeter.
with headphones connected to the headphones output on the DJ controller.
2) I recommend to look at the videos tutorial suite
to avoid returning a DJ controller fully working when you do not understand a feature, if you are interested in Djing.