The output settings for the sampler sound may not be correct.
Make sure you're running the latest firmware version and check the following points:

1. Ensure that [A] is selected for [OUTPUT] on the upper right of the sampler panel in Serato DJ Pro.
2. Use the [SAMPLER VOLUME] knob to adjust the volume.

For details on step 1, refer to the Serato DJ Pro software manual.
You can read the Serato DJ Pro software manual here [serato.com].