AU is an audio format standard in early web pages NEXT systems. It is also called u-law or Sparc-audio format. Typically, AU files can store audio data and annotation blocks. Initially, it was a headerless audio format, but newer versions have a header, thanks to the six unsigned 32.bit words.
FLAC (Free Lossless Audio Codec) is a free lossless digital audio format developed by Xiph.Org. A compressed FLAC file can be reduced by between 50-70% of the original size and decompress back to the original sound data. This file format supports metadata like ID3 tags, cover art, and album title.