In fruit ninja slash sound is played even if you keep finger on the screen after the slash (no released touch event) and also begins when the slash start ... so perhaps it's more something about velocity checking ... and so need checking the range between previous and actual touch X/Y to give short or long sound ! it's not so easy and it's perhaps done with a lot of very small sounds playing after each others or a single sound but changing the playing speed of the sound itself !
Good luck, and i think many guys need this kind of effect so it's a really interesting post ;-p