The basic Idea was to have Christmas lights flash with the music. In my design I used an ordinary amplified computer speaker, a diode bridge, and a 'CRYDOM' SSR (Solid State Relay). In order to increase the time that the lights were on as well as protect the SSR I used a Diode Bridge to rectify the signal from the amplifier circuit.
Things that you will need:
1. Solid State Relay
2. 4 diodes
3. Computer speakers (or some sort of audio amplifier)
5. AC Outlet
6. Various wires
Large Hammer (very important)
Step 1: Finding a Solid State Relay
Any SSR can be used as long as it is rated for 120VAC or greater and the correct amount of current. A standard strand of 100 Christmas lights is ~0.5 amps. To be on the safe side if you only wanted to run two strands of 100 lights you should probably use a SSR rated at ~2 amps (more is always better in this case).
Something else to keep in mind is the input voltage range. 2-32 VDC is the most popular range (I find that this range works best).