Lampdog
You can turn it up in trim mode.
Press Shift + Trim
Press Perams and use the jog wheel to increase level.
I think its always a bit louder in in trim mode than when you are in program mode.
So maybe if it is still too quite you could turn down the volume of other pads by pressing shift
+ mixer to compensate.
You can also turn up the master level of the mpc in the mixer window by pressing Setup.
Or just sample it again louder.
Even when the record level seems to be peaking you still get a good/sometimes better recording.