I meant calibrate the SoundCardOsc's Y sensitivity to +1V at the top and -1V at the bottom of its screen, if it makes any difference!
I've tried it as an experiment. I must have been using the mic input because the SCO flat topped at only +/-0.5V, - long before the 4 diodes would conduct, so I decided to just tap the 35mm piezo very gently to keep it's output below 1Vpp and get some results. It was easy to get plenty of good captures.
For proper finger taps I think you'd need to use a 20:1 potential divider, and for hitting the piezo with wooden sticks 200:1.