I just trimmed a short sound and now the MPC won't play it.

If you trim a sample to less than 1368 samples (about 30 ms) the MPC will not play it.   The get it play, add enough silence to get the sample length longer than 1368.   It doesn't have to be exactly, just greater than 1368.

Why?  The MPC automatically decays off the last 30 milliseconds of a sample to prevent clicking.    If you haven't noticed, when you slice a sound on the MPC2000XL, the MPC adds samples at the end of each of these slices so that every slice will play when triggered.  I discovered that the MPC will add milliseconds (30 is the default, strangely enough) to the ends of each slice.  The manual wasn't clear on what it added (samples or msecs), but it is msecs.