It is possible to play a song with iTunes or other software giving the input with a sensor connected to the Arduino board?
How can I do?
please someone help me.... thankyou 
Hello Alessia
I think on windows it can be done with a winamp plugin.
If you asked for Linux , that whould be quite easy with mpd (MusicPlayerDemon)
http://www.musicpd.org
Eberhard
THANKYOU Eberhard ...
really I asked for mac... but I understand that I need a Music Player that allows remote access for playing music... maybe it is too difficult for me (now)...
A. 
Don't give up so quickly, there are many guru's that will be glad to help!
You can control iTunes with Applescript - you would need a program that was responding to the arduino messages and setting off applescripts.
If you get arduino to send midi info and hook it up to a midi to usb adapter you could use midipipe which is free and can carry out applescripts in reponse to midi notes.
If you use Max there are externals called Tap.Tools that include an applescript external that will run applescripts.
You could use programs like Max, Pure Data or Processing to play the audio files directly rather than having lots of intermediary program, depending on the situation.
HTH
Nick
I agree with Nick; something involving applescript is probably your best bet.
There's also a program called SerialMagic that essentially uses serial data to trigger keystrokes. I'm not entirely sure how this program works as I am not at my mac right now, but it seems that it basically coverts incoming serial data to keystrokes. It seems like you could either bind applescripts to Fkeys or just have the Itunes open and just have SerialMagic jam on the left or right keyboard keys as data comes in from Arduino.
Unfortunately the program is 40$ and I don't know how functional the demo is or whether it would actually work with Arduino. Some experimentation by someone actually near a Mac is in order.
More info is here:
http://www.serialio.com/products/SerialMagic/index.php
FYI I'm currently trying to do something similar, only instead of sending serial into the computer and having it respond, I want to send serial data out to the Arduino board (in my case, I want to display my unread mail count via LEDs). Topic for a post of my own once I run out of research leads 
hello
she got it to work with this http://tinker.it/now/2007/04/26/control-your-mac-from-arduino-the-easy-way/
(from the italian forum)
There is also a $2.00 plug for an ipod that allows control of the whole interface. So if you don't need anything else on your desktop machine, and you own an ipod, that might be a way to go too.
Paul Badger