Generate audio file

Hi all,

I want to build an app where people can download an audio file.
The goal il to create something like a remix of a song, and users could download their remix.
I found some exemples where the record is done with microphone, but the result is disgusting. I saw several app like Groovepad: who does what I want, but I have no idea how they did.

If someone has an idea…

Thanks by advance !