When I move the crossfader to the side where a deck is playing, no sound is output. Why not?

The crossfader settings may not be set in the way you want the crossfader to work.

・Check the crossfader reverse setting.
If the [REVERSE] switch on the front of the DJM-S7 is set to [ON], the operating direction of the crossfader is reversed. In this case, set the switch to [OFF].

You can read the Instruction Manual [here].