Do you have the 5 volt fan and mp3 player wired to power on when power is applied? If so, then to get what you want you'll need to connect them into the circuit differently. Based upon how/when you want the devices to be energized..
How about a wiring diagram and a better explanation of what's going on and what you want to go on. How are you wanting to control the 5V fan and the MP3 player?