Is it possible to connect Arduino with an MP3 player and how can i select the soundtrack that i want to play?I want the soundtrack starts to play when magnetic sensors come to a contact.
Can somebody help me? Please...... 
I've been interfacing with this MP3 player which is pretty good:
http://www.vinculum.com/prd_vmusic1.html
I'll upload my code to the playground this week, but it's quite straight forward, you talk to it via a serial connection, then you can controll track selection, play, stop etc. I haven't worked out yet how to control volume yet though
If you don't want to buy the mp3 player above I'm sure you could hack an ipod, i know sparkfun have some connectors but not sure what functionailty they have:
http://ipodlinux.org/Apple_Accessory_Protocol
hope this helps