Taken from wikipedia
The AU file format is a simple audio file format that consists of a header of 6 32-bit words and then the data (high-order byte comes first). The format was introduced by Sun Microsystems.
The type of encoding depends on the value of the 'encoding' field (word 3 of the header). Formats 2–7 are uncompressed PCM. Formats 23–36 are ADPCM. Formats 1 and 27 are μ-law and A-law, respectively. Several of the others are DSP commands or data, designed to be processed by the NeXT MusicKit software.