You could have the arduino control what is played from a card like thishttp://www.mdfly.com/index.php?main_page=product_info&cPath=9_53&products_id=284
That looks an interesting card, the one from AdaFruit also has the audio amplifier, volume control and speaker but maybe he needs to have a more powerful one.http://www.adafruit.com/products/94
I've also found this shield, any opinion? https://www.sparkfun.com/products/9715
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