The file extension AU is used for audio files in AU format. It is the standard audio format on Sun and NeXT computer systems. The AU format is a container format that can include, but is not limited to, uncompressed LPCM (Linear Pulse Code Modulation) audio in mono or stereo, as well as various compressed data types.
The most commonly used audio format for AU files is compressed 8-bit μ-law.
The audio editing program Audacity can also handle AU files when ffmpeg is installed. It should be noted that many programs can not handle all data types of the AU format.