As stated previously, this step should be done in tandem with the previous steps as they are intrinsically tied together. This step requires you to do in-depth and highly detailed research into what you wrote for the basic idea/concept. Not only are you looking to see if what you want to do is possible, but also whether or not the language, game engine, or library/ framework you want to use supports it. This also requires you to know which platform you want to release on.
With the basic idea of this application, we know that it would require utilizing the camera and having touch events to detect whether something is lit well enough or not to determine whether or not it should play the audio file.