"There are plenty of online guides for making contact microphones from piezo discs to listen to the microscopic vibrations of objects. (Steve Schaberg's guide is great for first-time builders, as is Furious Contact Mic Assembly.)
My version includes a limiter to protect sensitive mic inputs from high-voltage spikes. There are many better designs, including fully buffered FET preamps, but if you're looking for a "Radio Shack" solution hat won't fry sensitive mic inputs, this will do!"