Some audio files in M4A format do not play on some devices.
Remuxing reassembles the data packets of the audio stream and that’s exactly how to fix the problem.
FFmpeg | How to “Fix” M4a Audio Files
1. Install FFmpeg (Windows version)
I dumped the FFmpeg.exe and FFprobe.exe into a directory “C:\OpenPaths” on my main drive. If your FF files are someplace else, change the paths accordingly.
2. Convert Single File
Enter this on the command line:
C:\OpenPaths\ffmpeg.exe -i myAudioFile.m4a -c copy myAudioFile_REMUX.m4a
Again, C:\OpenPaths\ is where you have your ffmpeg.exe on you computer.
3. Convert Multiple Files in Directory
Create the FolderM4AtoM4Aremux.bat file. All you need to do is to copy the text below into a notepad or equivalent text editor and save it with the .bat extension.
For /R %thisdir% %%J IN (*.m4a) do (
"C:\OpenPaths\ffmpeg.exe" -i "%%J" -c copy "%%J"_REMUX.m4a
Change C:\OpenPaths\ to where you have your ffmpeg.exe on your computer.
Create a new folder (e.g. M4A) and put all the problem M4As into it.
Now, from the Explorer window, drag the folder (M4A) with your mouse to the top of the batch file icon FolderMP4toM4A.bat.
The command/terminal window should open and show the conversions.
The new files in the same directory that have the _REMUX are the “fixed” files and should now be good to play.