Sorry for the lack of knowledge here:

I have sampled a poem being read and want to put it over a beat. When I play it over the beat, the vocal sample is slowed down and sounds choppy. How can I fix this?

Try to do this:
1. select the pad where is the sampled poem
2 press the time/bpm button
3 turn the knob control 1 and see if its in "PTN" position, if so, turn all the way down until de display shows "OFF"
I think it must solve your problem, but it can be a polyphony issue too.