Change voice during phone call android

view story

http://stackoverflow.com – I want to make an android application that allow user change the voice during phone call. For example: You are a man, you can change the voice to a woman or robot when talking over phone. It is like a funny prank. I work around android's api and google for some days but still have no idea. Some one told is impossible but I see some app on google play can do: https://play.google.com/store/apps/details?id=com.gridmob.android.funnyc... So I think there are someways to do that. I think about recording and play back by using AudioTracker but I have 2more problem: 1. I cannot mute the vo (HowTos)