Great holiday Instructable! “Use solid-state relays to blink the lights on your tree in time with music. String lots of lights onto the tree. Use as many sets as you like, but arrange them such that there are three light regions, each with a separate plug. Run these three plugs down the trunk so they can easily be attached to the light controller. Up to three strings of lights can usually be linked in series, so you can almost certainly cover even big trees. Plug light strings into the controller you built for Halloween (MAKE 03). Attach computer and sound system…” Link.
HOW TO – Computer-controlled music-synchronized flashing tree lights