Sure, first thing you need to do is work out whats a “high” signal on your piezo sounder. Use a voltmeter or oscilloscope to find out what kind of voltage you want to perceive as high.
Let say you find out that the voltage your piezo outputs as “high” is 0.5V,
You now need to get yourself an op-amp (I would suggest LM193 as a decent allrounder), connect the output of your piezo to its noninverting input (symbol +) via a 10k resistor , and supply the inverted input (symbol -) with 0.5V.
Supply the LM293 with VCC = 5V, and have a potential divider of 9K/1K (or any values to that effect) producing the 0.5V you need. Pull the output up to 5V by a 3k resistor, and connect the output to the noninverting input via a 10meg resistor.
What the op amp will do is take the difference between inverting and noninverting input and amplyfies it. If noninverting is greater than inverting, it will just output positive, giving us no change, if it inverting is greater than noninverting, it will output 0V, giving us a low signal.
Op amps are great little devices :).