‘m4a’ files are converted via the Quicktime module installed on your computer, if any.
First think I would try, would be to update to the latest Quicktime version for Windows.
If you convert the files one by one (drop one onto WaveLab), do you get the problem?
Also, did you try with the batch processor, or batch file convertor?
(with aplogies forthe slow response)
1 - Yes.
2 - Yes.
3 - Not yet - will try this later.
FWIW, the same files convert properly & painlessly in a $50 suite called “AVS4U” but as soon as I try in WL, I get the C++ wavelab.exe crash