I built a laser piano using LDRs and laser light diodes. I used an Arduino Leonardo Pro Micro connected to a speaker to generate sound. 

There are five steps to make this project : 
First Step: Marking, Cutting and Gluing base 
Second Step: Inserting and attaching LDR and Laser diode in the cardboard 
Third Step: Soldering 
Making the Circuit 
Coding 

I have given rest of the details here :

https://hackaday.io/project/27646-diy-laser-piano