What is the difference between an .aac file and an .m4a file? If I just change the file extension of an audio file recorded and then converted into "AAC" between .aac/.m4a it drastically affects how the file is treated.
AAC: Plays in a web browser, not on the iPad M4A: Plays on the iPad, not on a web browser.
Is there a real conversion between the two formats?
Sometimes the file extension refers to the wrapper of the file. A codec like aac can be wrapped in more than one file format, for example 3gp, mp4, or adts.
I haven't tried directly changing the extensions of .aac and .m4a files from one to the other, but from past experience, I've had encounters with audio encoded via AAC that, when played as an AAC file, would not display the total time of the track in my media player. When wrapped in an m4a file using an mp4 muxer, and played as such, the total track time would then be displayed. So it does have an effect.
And as mark.chackerian said, the file extension can refer to a wrapper format and not just the codec used to encode the audio. AAC is the codec, MP4 is the wrapper format, and .m4a is the extension for audio-only MP4 files (I believe).
.m4a ought to work on both -- have you checked that your web server is configured to send the correct MIME type audio/mp4for .m4a?