Need help with coordinating lightshow with sound

I've found that it's easier to use a Raspberry pi for projects involving sounds because it has built in audio suppport.

For Arduino, you could purchase an MP3 player board, or have it listen for a command from the computer over the serial port. Computer would send it a command to activate the LED and then play the music itself. I can't think of example sketches that do this, but it's not that hard to figure out.