AU (Sun Audio) is an audio container format originally developed by Sun Microsystems and introduced to the broader audience with their Sun Operating System (SunOS). AU files were a common encounter on SPARC and NeXT computer workstations, as well as on some of the early websites. Today, the AU file format is standardly supported on Unix-based platforms and Java.

Initially, AU files were really plain and simple, storing only a basic µ-law-encoded representation of the audio data inside them. However, later on, the AU specification was upgraded to support a wider array of audio data encodings, including various PCM, IEEE floating point and ADPCM representations of the data. A-law encoding support was also added.

Nowadays, the AU file format is considered a bit outdated and therefore many modern digital audio players and desktop audio applications cannot properly recognize and play AU files, which is a major disadvantage. Fortunately, there are plenty of free applications available that allow users to easily convert AU files to more common file types like WAV for example.

Sun Microsystems AU files should not be confused with Audacity AU files. Audacity is a popular free audio editing application that is also cross-platform. When you import audio data into your Audacity project, this data is automatically saved as AU files when you save your project. The AU files in this case represent Audacity Audio Blocks rather than a Sun Audio files, and are therefore in no way compatible with the applications capable of providing playback to standard AU files.