What is the difference between M4A and AAC Audio Files?
.M4A files typically contain audio only and are formatted as MPEG-4 Part 14 files (.MP4 container). .AAC is not a container format and instead it is a raw MPEG-4 Part 3 bitstream with audio stream encoded. Note that M4A does not have to contain exactly AAC audio, there are other valid options as well.