Yes, something similar. There was that symbian app that added sound effects to your voice to make the other side think you are in the street, or a musical concert or something while you may be in your room. I want to do something like that.
Also I was thinking of making an answer machine app, which can pick the call and play a sound file which the other end listens to.
But that all depends on being able to send your sounds over the cellular network.