What sdrawkcab25 said. I would imagine you'd have to pay for using it, and depending on the song, it could be very expensive. Music rights are big business these days.
However, if you do need music for an app, you could always do a Google search for royalty-free music. Terms will vary, and some artists will allow free use for any purpose, for non-commercial use only (not a paid app) or whatever other terms. This could get pretty tricky.
Good luck!
