Well, you made be do it.
I dug out a couple of piezo elements I had lying around. One of them is in a housing like the one in the OP but PC mount. It produced about 1Vpk-pk when I blew in the hole. The other one I found was "naked" and produced around 10Vpk when lightly tapped with my finger. So your intuition is right; it'll take a pretty loud noise to trigger the SCR, but mweels did seem to want to detect loud sounds:
A multiturn POT would make it much easier to adjust the sensitivity.