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

Serato DJ Pro sampler settings might not be set correctly.
Check the following:
(1) [A] is selected for [OUTPUT] in the upper right corner of the Sampler panel.
(2) Sampler Volume is set to an appropriate level.
(3) Sampler Mute is set to off.

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