Already built iOS music app feature fix - repost 2

Hi! Please only bid on this project if you have the necessary skill set and are able to communicate well! I have one completed iOS music player app, except for one sound altering feature that I need someone skilled in music and coding to help make the way I need. The feature is to change the tempo of the music by slider between 80% and 120%. But the sound quality and flow has to be maintained. And you need to be able to change the music 1% at a time. The project app is complete, but I just need someone to re-amp the code that surrounds the tempo change, if that is at all possible. If you believe you might have sufficient skill set, message me and I can give more details about the project.