M4A files have aac audio content and mpeg ll video content as stills, or slides that come in on time markers. Online courses, language courses use them and just basically recommend iTunes to play them. This has some disadvantages as below:
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.
It has taken me a year to find the solution and I want to share it. If I have maligned any Apple products, please correct me. The Apple App store provides
MPlayerX, an aftermarket and small mimic of Quicktime, similar to VLC Media Player. VLC does not play the slides, only the audio. MPlayerX will play the slides LARGE by default, but only if you make a small change to the file extension: from .m4a to m4v. Then it sees the slides as "video". FYI the way I use to do that is Control click/Get Info and use the File name and Extension field.
I recommend this because I can play these lectures, with slides large, access them easily and keep them out of my iTunes library and in fact on an slick not on my HDrive. Now I just want a way now to play them on my Android Tablet and I will be set.
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. Roger