These days there are a number of audio synthesizers which do basically what you said, such as the Beepmap plugin that comes stock with FL Studio.

Among the first to use the technique artistically, Aphex Twin famously drew his own face in the spectrum of his track "Equation" (https://www.youtube.com/watch?v=wSYAZnQmffg - skip to 5:20 for the face).

Here's a few more, if you're interested: http://twistedsifter.com/2013/01/hidden-images-embedded-into...