kit 74 parallel relay board and Vixen Lights software. Kit will be around $30 some bucks and Vixen lights is free. Most expensive thing will be your time to create the sequences Let me know if I can help.
Lightning FX box. Getting harder and harder to find them tho. The halloween version is black with an orange lightning bolt and the x-mas version is white with a x-mas tree - they both work exactly the same tho and it doesn't matter what the case looks like so if you find the x-mas version for cheap - get it.
Simple - plug in lights and put it next to a sound source.
The lighting FX box will make the lights flicker or flash but not in sync to music like I think you are asking about. This system operates solely by the sound level of the input and it has a volume control to adjust the sensitivity to the sound level.
What are you trying to do with this device? a singing pumpkin routine by any chance? if so, cheapest and easiest way to come close to this effect would be with a Gemmy Light Show with an mp3 plug in. An LOR system would be a more sophisticated and more expensive alternative with the best syncing. You'll find a number of threads on the forum about both. Depends on your budget and abilities to program the system (particularly the LOR; Gemmy LS, just plug and play--results will depend on the songs used--it's pretty much a stand alone system with no need to connect to a computer unlike the LOR).
Sometimes you can find the Gemmy system at very good prices. Xmas stuff is always cheaper than halloween versions; greater volume, lower prices. If you will use an mp3 player, then the preloaded songs won't matter much and I'd look for the best price you can find. Be aware if you see a great deal in price that there are versions of the xmas green LS that did not have an mp3 player attachment and you wouldn't be able to use it for your halloween usage.
The lightning FX box would probably be the easiest, but they're getting hard to find. A color organ would do the same thing. If you want more accurate syncing, the Kit74 / Vixen option works well if you can dedicate a PC to the show.