■rekordbox
The output level of the sampler sound in rekordbox (Performance mode) might be weak.
Select [A] for [OUTPUT] in the lower left corner of the sampler deck.
Set the [GAIN] to 12 o'clock in the center of the sampler deck.
Check the [SAMPLER VOLUME] knob on the DJM-S7 is not set to -∞.

For details, see the rekordbox Instruction Manual [here].

■Serato DJ Pro
Serato DJ Pro sampler audio routing may not be set correctly.
Check that [A] is selected in the [OUTPUT] in the upper right corner of the sampler panel. Check the [SAMPLER VOLUME] knob on the DJM-S7 is not set to -∞.

For details, see the Serato DJ Pro Manual [here].

Sampler mode

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I can't hear the Sampler sound. Why not?