As I hadn't yet done so, I've now checked the source code for how it handles the installation of the ACC 'plugin'.
Essentially, it modifies exec.ini and adds 3 entries:
Call ExecINI.WriteString("Phatbox", "player.m4a", "/dos/aacplay")
Call ExecINI.WriteString("Phatbox", "player.aac", "/dos/aacplay")
Call ExecINI.WriteString("Phatbox", "player.mp4", "/dos/aacplay")
Out of interest.... what is the exact casing of the extension on your files? Could it be the extensions are upper case, and it's not matching a player because of that?