MKV is a container format for holding multiple audio, video, picture, and text data such as subtitle tracks. It is similar to other common containers like MP4 and AVI, although MKV is open in the specification. For video contents, MKV uses a .mkv filename extension, .mka for audios, and .mks for subtitles.
MPC-HC, KMPlayer VLC Media Player MKV File Player MPV
AA (Audible Audio) is an audio container format with a .aa filename extension and is used to store audiobooks. It can contain audiobook features such as spoken-word audio, bookmarks, chapter markers, and more. This format is supported by portable devices like Apple iPod, Amazon Kindle, ScanDisk Sansa, Creative Zen, and more.
Apple iTunes, TuneCab Nero 2019 Audible AudibleManager Audials Tubebite 2019