There is only one file type that uses the .NAAC extension:
Namco AAC File
An NAAC file is an audio file used by Nintendo 3DS games developed by Namco, such as Ace Combat: Assault Horizon Legacy and Taiko no Tatsujin: Don to Katsu no Jikuu Daibouken. It stores compressed audio for a Namco 3DS game in the Advanced Audio Coding (AAC) format. NAAC files can be opened using vgmstream.
Updated: September 18, 2018