Step 1: The Schematic and Parts
2 10k resistors
1 100k resistor
1 electret condenser microphone (Click here to get it at Radioshack)
2 0.1uf capacitors
1 AG12 4 battery coin cell holder
-jacks and connectors that will help you connect your microphone to an external speaker
The schematic is pretty straightforward. The electret microphone, as seen in one of the pictures, has 3 small solder joints that connect one of the points to the outside casing. This is the negative lead. If you happen to find a 2SC6067 transistor, congratulations! If not, use any small NPN transistor. Keep the board and parts as small as possible. You can remove the battery holder off of a dollar store burglar alarm, which is what I did. If you do decide to use an Altoids tin, make sure to clean off burrs after drilling holes to avoid cutting yourself. Also, put some tape on the inside of the Altoids tin to prevent shorting.
Go ahead and solder!