Connect an mp3 player to an Arduino? I have been looking for a good way to play sounds using an Arduino.  I started to consider just buying a cheap mp3 player and replacing the player's buttons with optical switches, which connect to an Arduino.  I wanted to know if anyone has done this before and if there was a good mp3 player for this (one that may have some sort of control port / ir sensor.)
Yes, people have done this before, and they have documented it on their own blogs and similar sites.  Maybe you should have asked Google first before you came here?  You'd most likely have ended up on instructables, where someone has documented exactly what you want to do and how to do it. and saved yourself this entire step.