this will be hard to explain if u not fully a customed to the nnxt.
click on the individual sample and select the volume u want it to play at in the panel below. now load up another sample, and do the same. etc.
as for velocity... just press it harder or softer..
for pitch. the automapping does that for u. or u can select the individual sample and pitch it up or down, and even fine tune it. just read all the knobs