podPress does not include a player for .m4a files (only for .mp3, .ogg, .swf, .flv). It relies on browser plugins like the QuickTime e.g. for the m4a files and shows a preview image first.
Some browsers like Chrome and Safari will show a play button and use the HTML5 player of the browsers to play .m4a files. (In order to achieve this you would need to activate the option “Use HTML5 tags:” on the podPress Player Settings page.)