If you wanted to play an audio file with 3G/GPRS Shield for Arduino (3G+GPS), you had to convert it (mp3, ogg, wav...) to an amr/qcp file using a software like Audacity.
Now you can upgrade your firmware with this file in order to play mp3 directly. Connect your 3G shield to your PC and run the program. Follow the steps and you will update your module. You must use Windows to run the program (it doesn't support and emulated Windows).
Check our tutorial to know how to use this shield.