need an app for IOS to mix tracks.

hi all,
I am needing an app for IOS that will let me mix tracks. I know of Garage Band, But I would like something that will let me load a prerecorded track, and that works with dropbox. I am willing to pay for something, but I can't at the moment, so if there is a free app that will do it for now that would be great to. Anyone have any ideas? Thanks.
Cat