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="" id="id_353015" play="false" ]

[pix_audio src="" id="id_353015" play="false"]