The following circuit is not from me but from Burkhard Kainka. I have redrawn it and modified it a bit. So I use only one voltage and I added a MOSFET switch. It is basically a bat detector, but you can make many other noise sources audible with it. The interesting thing about the circuit is that an AM/FM radio chip (CD2003) is used in the core.

PCB layout top:

Bottom:

PCBs: