mp3 is a lossy format. It works by modeling the acoustic response of a person and throwing away information that it considers that a person can't hear. In essence louder instruments that overshadowing soft instruments - the softer one gets ditched. This means that you never will be able to extract what originally went into the mp3 let alone separate out the instruments.
I have a friend who is a sound engineer and he always has to say "No" to people who ask him to re-engineer a track recorded as an mp3.
A few weeks ago I saw a study that suggested that the younger generation actually preferred the sound of mp3's over more complete formats as that is what they had grown up with.
