this is open lossless codecs, developed under the Ogg project. Lately it’s become quite popular: many users wishing to store audio with original quality choose FLAC. This is partly in connection with the fact that codecs uses IDv1 and IDv3 tags - this allows a user to sort an audio collection according to one’s particular tastes, changing the original metadata of the tracks.
AU is a file extension for a sound file format belonging to Sun, NeXT and DEC and used in UNIX. The AU file format is also known as the Sparc-audio or u-law fomat. AU files contain three parts: the audio data and text for a header (containing 24 bytes) and an annotation block.