How to figure out the notes of a piano piece in an mp3?

If you want to play by ear, the best way I've found, especially for something complicated, is to learn it phrase by phrase, replaying each phrase over and over again until you have it down, then moving on. If you have an audio editing program like Audacity (which is free), or better something like Pro Tools, Cubase, etc, you can slow it down and loop individual sections to make it easier to figure it out. That's what I do.