AndertonAudioSnap usually works for me if I set the Threshold etc. correctly, although sometimes I need to move the transient markers around a little bit to catch the very beginnings of impulses. However, there's another option where you basically set the on and off levels where you want to split the clip. It's under Process > Apply Effect > Remove Silence. The screen shot shows separating multiple kick drum hits that I recorded at various velocities for an upcoming Drum Replacer Expansion Pack.
