This 2X2 LEGO brick has a built in red and green led. It is controlled from the motor ports of theRCX. When the port is turned on, depending on the polarity either the green or the red LED goes off.

The following project is a sound sensor that runs on the sensor port of the RCX. It uses the basic circuit described at http://ourworld.compuserve.com/homepages/Michael_Gasperi/lego.htm by Michael Gasperi with some changes. It has a common electrec microphone and a pre-amplifier. The sensitivity can be adjusted with the 1K potentiometer. On a highest set a soft whisper can trigger the RCX. The circuit is built on a universal printed circuit board and embedded in clear casting resin. The bottom of the cube is a 3X6 LEGO plate. The components can be obtained from the local Radio Shack.