An MP3 file is a final product of recording from different real or virtual instruments. Once put together it cannot be taken apart into original pieces which means that you can't transcribe the file into sheet music but there is a way which is a bit complicated with the help of Transcribe!. The software analyzes the MP3 file and slows it down and tries to guess the notes. From there you can perform manual writing of the notes, tempo, pitch, etc. I believe this is the only way to transform an audio file to sheet music.