Have you recorded something on your phones voice recorder and now the file won’t open? This worked for me! I am super happy to have found this video!!!
My twist on his video is that I opened a working m4A file and the corrupt one in the Hex Editor and copied the code from mdat forward (that he deleted) from the working file to the not working file.
When it saved it saved as a WAV file which I then opened in Adobe Audition and saved a mp3 which was my target format anyway.
I hope this helps you as it did me:
His Hex Editor download link is: https://mh-nexus.de/en/downloads.php?product=HxD
Please check the links in the description of his video if you choose to go through the rest of the decode /encode process.
He also has a Paypal donation link if you are as grateful as me:
Support channel & make donation : https://www.paypal.me/aminenina/5