For sensing hammer impacts a piezo sensor with the correct circuit to sense the magnitude of force by the voltage rate of rise. might work. The other scheme will be a section of polarized PVDF film over the wires.

 

18 days later it seems that a contact microphone on the back surface of the sounding board is the way to go. OR the piezo disk sensor, probably several of them , feeding a low noise amplifier. Adjusting the gain for each pickup should be able to provide a uniform response.

 

If a capacitance sensor is made part of an oscillator circuit, the change of capacitance from an impact would cause a change in frequency proportional to the deformation of the capacitor.