This is a dual mic (left/right) input sound sensor. I made this sound sensor to add to a robot for ears. The heart of the circuit is a LM324 quad op amp and a pair of condenser mics.
The sound sensor operates by taking mic signal into an Op amp + input to amplify it. The output from this first Op amp is inverted by a second Op amp by feeding the amped output into the - input of the second Op amp. The output from the second Op amp will only go high when the mic detects high level of sounds.
This sound sensor circuit can be altered simply and used as a pre amp for the mics.

Step 1: Breadboarding

I tried several condenser microphone circuits and was not happy with the results.
I wanted a sound sensor that would output voltage when it heard relatively loud noises. I.e. a decimeter. It couldn't be too sensitive it needed to "ignore" background sounds and only pick up local noises like command sounds.
The sound sensor also needed to be 2 input left an right "ears" so I could compare which direction the sound was coming if desired.
The work done by diybloke sent me on the right direction. My final circuit is very close to his.

Step 2: The Schematic

I found using 2 op amp to condition the mic signal if only the + inputs on the amp were used yielded a pretty good amp sound of what the mic was hearing but I didn't see good voltage changes that were usable. But feeding the output of the first amp into the invert side of the second amp gave me the voltage change I wanted although the sound quality of the final output was not great. But for my purpose this didn't matter.

The first schematic pic above is actually 1/2 of the final sensor but ill start with this for simplicity.

Parts needed:
Op amp x 2
I'm using 1/2 of a LM324
Condenser mic x 1
Resistors
100K x 2
1k x 2
22k x 1
68k x 1
Capacitors
22uf x 1
470uf x 1
0.1 X 1

In the second pic is the complete sound sensor circuit with LED displaying the signal outputs from the 2 channels.

Step 3: Soldering

Above are shots of how I layed the circuit out on the Project board. After testing that I didn't mess anything up too bad the LEDs were removed and another pair of terminal connectors was added to output to my logic display and elsewhere to use the signals.

Done!!!

Step 4: Final Notes

For final mounting in my ongoing robot build A3-8D I stacked this sound sensor with a binary counter circuit and mounted this pair on a scrap price of lexan plastic.
The pair of boards on the left of the first pic above are audio amp circuits salvaged from thrift store phone dock speaker units. These were simply mounted to the radio shack PCB I like #276-150. The small circuit piggybacked on the lexan mount is a 12V to 5V regulator.

The mics and their mounting are seen in the last pics. These were mounted on either side of the robot's head. For the time being the sensor only indicates with LEDs when it hears something I plan to use these outputs as feedback to robots arduino processor.