You are describing a LIGHT ORGAN. This is a device that modulates the power to one or more lights in response to audio input to the device. While the basic components of such a device are relatively simple and straightforward, I believe that your best bet for getting one up and running by Halloween is to buy a light organ kit. There are lots of them out there. Google "light organ kit," and you get more than 3 million hits.
Watch out for kits that require input from an audio power amplifier. This is the most common configuration due to the primary application of such kits. Since I gather that you want the light organ to be driven from a microphone, you would need a microphone preamplifier to drive such a unit.
Since you are not looking for multiple colors controlled by different portions of the spectrum, which complicates the device, pick one described as a, "single channel" kit. A single channel light organ drives only one lamp or cluster of lamps or LEDs in response to a single audio signal. Multiple channel light organs control multiple light sources of different colors from different portions of the audio spectrum. I gather that you are not looking for that type.
You can get single-channel light organ kits in the $5 to $50 range. Velleman is a very good kit maker. Here's a $5.95 Velleman kit (http://store.qkits.com/moreinfo.cfm/MK103) that powers 4 red LEDs mounted on the board and has a microphone built in on the board. Looks perfect for your project if you wanted the microphone inside the pumpkin with the light source. Consider substituting ultra-bright white LEDs for the red LEDs provided with the kit. You can get the ultra-bright LEDs from Digi-Key or Mouser or possibly your local Radio Shack. You will need a power supply in the range of 8 volts to 15 volts DC. Since the kit consumes from 0.5 to 20 milliamps (depending upon the instantaneous intensity of the light), you could probably get by for a few hours on a 9 volt alkaline "transistor" battery, which would make the whole thing self-contained with no power cord. You can conserve battery power by adjusting sensitivity to avoid having the LEDs on most of the time.
Have fun.
awright
