Okay, so after much trial and error it was super easy to get the audio working in obs with my traktor S4. On either of your main outs plug them into a new channel. C or D, either or. Make sure you switch them to line in mode. Okay, now in windows go to your audio device manager, go to recording and choose the line in routing you chose for your traktor s4, Channel C or D, make it default. Now in OBS go to audio settings in under Microphone/LineIn choose the audio channel thats lined in to your s4 or default. Volume should come out now, though its under Microphone volume in obs. Good luck! Pm for questions.