The Channel A.
2.2.3 If You Don’t Hear the Track
 
If  the  track  is  playing  but  the  audio  coming  through  your  amplification  system  is  not  loud
enough, or if there is no sound at all, check the following:
Using Your S5—Getting Started
Playing Your First Track
 
TRAKTOR KONTROL S5 - Manual - 27