how to convert m4a to AIFF?

Firstly, go to iTunes Preferences>General and click the 'Import Settings' button. Set 'Import Using' to AIFF Encoder'.

Click OK, then OK again to close the Preferences.

Go to the Music list; select the files you want to convert and from the 'File' menu choose 'Create new version' then 'Create AIFF version' (iTunes 11 - in earlier versions choose 'Create AIFF version' from the 'Advanced' menu).

AIFF versions will be created and placed in the music list (the AAC versions will remain there). You can drag them out if you want to save them elsewhere (doing this will not remove them from the Library). You can select and delete any files as required in the Music list. (Never tamper with the actual library in the Finder).

However, if you are doing this because you think it will improve the quality, it won't. All it will do is make the files larger. M4a compression works by leaving out some of the sounds on the assumption that for psycho-acoustic reasons your ear won't hear them anyway. Once this is done the missing audio can never be restored: converting to AIFF simply reproduces the same compressed sound in a larger format.

Set the Preferences in iTunes like this:

General > Import Settings "Import using AIFF Encoder".

Then select the song you want to convert, ctrl-click it, and select "Create AIFF version" from the contextual menu.

You're welcome. 🙂