Its full path is given as the first command-line argument. So you will want to add code to check to see if an mp3 file was supplied and autoplay it. How exactly you do it is up to you, but I recommend you read the documentation for TApplication.Initialize.
The following is an example, assuming you have an object named 'MyPlayer' with a method 'PlayFile', etc.
if ParamCount > 0
then MyPlayer.PlayFile( ParamStr( 1 ) );