To add to what John has already said...
M4A files have aac audio content and mpeg ll video content as stills, or slides that come in on time markers.
This is probably the source of your issue. An "Enhanced Podcast" is an M4A file containing AAC compressed audio with JPEG compressed still images. I.e. Apple apps (e.g., older versions of GarageBand) automatically convert supported still graphic images to JPEG during the export/share process which are then compatible with QT 7, QT X, QL, iTunes, etc. And, since I'm not sure what "MPEG II video content as stills" is supposed to mean here, I suspect this is what is causing your problems as a QT M4A container may not contain a "video" track, and, while it may contain a JPEG compressed still image "slideshow" track, it does not support other forms of image compression as far as QT is concerned.
M4a is native to Mac, iTunes and should play in Quicktime, but seems to be an orphaned file type and is poorly supported. I have a lot of lectures on online courses coming as m4a, with slides that play automatically with the teachers lecture. However Qucktime does not condescend to play my lectures or any of my movies and seems to be set up only to play purchesed movies from Apple. Also iTunes does play the slides, but only as a thumbnail and is sensitive-it disappears if you do anything.
To be QT playable, three conditions must be met:
- The file container must be compatible with the QT player being used. (In this case the M4A container is compatible with all QT based apps.)
- The data contained in the file must compatible with the QT player. (In this case MPEG-2 video would not be "natively" compatible with QT 7-based apps.)
- The QT app must be programmed to recognize and play the specific compressed data and file container combination. (In this case the video content is either being ignored since it is in an M4A file container or being ignored because the "still image" is in the wrong compression format.)
As to the display of "image" content, the size can vary for many different reasons. By default, a properly encoded "Enhanced Podcast" created by an Apple app would be limited to a 300x300 pixel area to conserve file storage space but this setting can be deselected if the user so desires. Using this option, images larger than 300x300 pixels are scaled so that the height is 300 pixels. If the width is greater an 300 pixels, then the width is cropped to 300 pixels. If the 300x300 option is deselected, then all images will be scaled to the "native" height of the first image and widths are cropped to the same dimension as needed. Third-party Podcast apps may allow other dimension/aspect settings. In addition, third-party apps that support the use of non-JPEG image compression formats may create preview JPEG "thumbnails" automatically as part of that app's workflow. Without a sample file to examine, it is difficult to tell what exactly is going on in your particular case.
If anyone has another method, please inform. I have found lots of people in similar position to me who can't get the slides to play. Chrome and Safari won't as far as I can tell.
Normally, the preferred method would be to create properly encoded "Enhanced Podcast" M4A files containing AAC audio and JPEG "slides" as images. You should also be advised that "Enhanced Podcasts" are normally delivered via RSS download and so most browsers don't "play" such content unless the webpage has been coded to specifically play the content. (E.g., the Safari QT plug-in automatically downloads the file if the browser is told to open the file's storage URL but will play the file if told to open a webpage coded to play the M4A file:
EXAMPLE 1: This URL points to an M4A file storage location
EXAMPLE 2: This URL points to an old iWeb page coded to play the M4A file
