I don't know what the exactly correct answer is (there may not be one), but this is all going to be down to codecs on the PC version. M4a is something that Macs handle inherently, and the codec for turning them into wav files and back again (this is the only file type that Audition handles) is part of Core Audio. Same does not apply to PCs, unfortunately. I think that the provision has been improved in CC, but it has also been suggested that unfortunately, the best way of handling this is to use an external converter...
Try Super - link at bottom of this page: http://www.erightsoft.com/S6Kg1.html
If you to open and edit your .m4a format file on adobe audition take this step.
1. Open your Adobe audition.
2. Go to Edit menu.
3. Go to Preferences.
4. Click-on Media & Disk cache
5. And click Enable DLMS Format support.
Now you can open and edit your m4a format file.