Think I’ve seen this elsewhere, but here is the solution incase others have similar issues.
Windows:Make sure you got the latest Denon DJ ASIO drivers. Restart.
Traktor:Go to controller settings, audio setup, make sure you have the ASIO / Denon device selected. Go to output settings, change mixing mode to Internal, then change the output routing. (master output and monitor) Audio channels 3&4 for master out, and audio 1&2 for cue. (Switch these round if the sound ain’t on the right channels). This is entirely from memory, so if it’s not 100% correct, shout, and I’ll check when I next have the lappy on.