Do you think Vex should make a sound sensor?
I do!
I thought they did have a sound sensor.
http://www.vexlabs.com/vex-robotics-ultrasonic-range-finder-kit.shtml
the ultrasonic sensor may use sound waves, but it is not actually sensing sound in the human range of hearing. it uses high frequency sound waves to determine distance by first emitting a wave and timing how long it takes to return back to the robot.
You can purchase a MIC at Radio Shack and connect it to one of the Vex Controller’s analog input ports. You may also need to condition the audio signal to within the 0 to 5 Volt range (gain) and also adjust the offset. This would allow you to recognize sounds including clapping your hands.
For voice recognition, you would need a Digital Signal Processor (DSP) such as dsPIC30F6014 from Microchip.