You can add an MP3 by using the button on the text editor (you can see it on the image above). You can also decide the autostart (play=”true”). Be sure to use always an ID, the ID must be always different. If you use the TinyMCE button the ID will be generated automatically.
[ pix_audio src="http://www.pixedelic.com/themes/enfinity/wp-content/uploads/2012/07/bouree_sample.mp3" id="id_353015" play="false" ]