Advanced sound sample playback on button push

I need an app that can play sound on button push, with lowest latency possible. Also if possible 1. The sound should be variable volume, depending on how strong the button push was. 2. Mix the sound, if button is pushed again, but the previous sound is still playing. This is kind of electric drum principle. Who can do that? Please give some details about other media/sound applications that you have done, or other iphone experience.