Heres the documentation and link from UpRev on how to make a head-phone human detectable knock sensor. The diagram is insanely easy to follow and the device even operates in stereo feeding input from Bank 1 and 2 knock sensors.
http://www.uprev.com/documentation/K...n%20Device.pdf
I going to build something like this using my Radioshack Mini Amplifier since I've alreay got it and a boat load of electronics. The mini amp also has headphone out in case I want to complete it into what UpRev posted in the above link, but the downside is I can only monitor one sensor bank at a time. The upside is this mini-amp has an incredible gain for a very high amplitude. This RS mini-amp is test equipment compared to the FIIO 5 posted in upRev. I'd think it might work better than the FIIO 5.
http://www.radioshack.com/product/in...ductId=2062620