Hi,
Hmm, IMHO that looks like the wrong type of "vibration" sensor. There are references to
earthquake, motorcycle/car theft and washine machine (drum unbalanced ?). The "sensor" looks a little like a mercury tilt swich (but probably using something cheaper and safer than mercury).
The circuit diagram lower down
in this link shows the sensor shunted by a 100nF capacitor and with a 10k pullup, so it's unlikely to respond to any "audio" frequencies.
As hippy and I have suggested several times now; if you're trying to detect a string of a musical instrument being struck, then the "obvious" method is an
audio detector. The waveform shown by hippy in post #106 looks eminently detectable.
Cheers, Alan.