load up ur sound in an NN-XT sampler, right above the "Pitch" section on the sampler there is a dial that says "Play Mode", which is currently set to FW, just turn it till it says BW on the display, job done
alternatively, just get FREE software of the name Audacity, which can reverse audio among other things. If you do it with the NN19, you'd have to mute the FW part of the sample until it goes BW
yea you can do all of the above.. or if you have Record you can load the .wav in as an audio file, and then right click the clip and select 'reverse clip'