There can be multiple reasons for not having sound into your headphones.
Just check these details to avoid any headphones problems.
Check that your headphone jack is firmly connected to your controller.
Check that the “Headphone” Volume knob is not at zero.
Check that one of the Headphones “preview” or “master” button in activated on the mixer.
To be able to make previewing, you will need to have a DJ Controller or Soundcard with 2 separate Stereo channels.
All Hercules DJ Controllers with integrated audio are equipped with multichannel stereo outputs.
You can also reset your audio settings to default by using DJUCED diagnostic tools.