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Ed
I'm sorry I didn't return here earlier. For some reason I missed the email notification of your last comment.
As Javier's linked page describes, your *.M4A file could potentially contain audio in a few entirely different encodings. There are a number of free tools around that provide very full details about how the audio and video streams are encoded in multimedia files but, when you consider the complexity of the content, I really don't think repairing them is something that you stand much chance of achieving unless you have a good and complete sample file and a lot of inside knowledge and experience of the different formats.
Attempting to transplant corrupt or missing header data from a good file in a hex editor is just a very small part of what could potentially be the problem, and you really have to know what bits and bytes belong/don't belong, and where each data section begins and ends. I think you made a valiant attempt with the hex editor though. Companies or individuals who have spent quite a number of years building up databases of commonly occurring data and from closely inspecting files are, I suppose, entitled to create a little bit of magic and mystery about their repair techniques even if the repair is a relatively simple one to them.
We could mess about for weeks and weeks doing trial and error copying and pasting in hex editors using dozens and dozens of backup files and possibly still have a completely unplayable M4A file, but on the other hand could just get incredibly lucky and accidentally paste the right data in the right place. I would guess that the former would probably be the most likely.
I am not in the position to personally recommend Javier's company software / web service over any other company's simply because I have never had to use such a service, but the company sites are well documented and I have seen that the founder has posted useful advice (along with the company link of course) in a number of help and multimedia-related forums.
Just so you are fully informed:
http://aeroquartet.com/company
http://www.simplemoviex.com/company.html
http://mp4repair.org/pwt6/faq.html#guys
No harm in making some enquiries.
Bill