Audacity Project Format
From Audacity Manual
Audacity project files store information about the entirety of the project - the number of tracks and their time positions, details of clips within the tracks, amplitude envelope points, labels and gain and pan data. In addition, "summary information" is stored which enables the display to be redrawn rapidly without Audacity examining the entire contents.
aup
Audacity projects have a master file that ends with the extension ".aup". In the same directory as the master file is a folder called "_data". This contains one or more subfolders with many small audio files in .au format of size 1 MB or less. The master file describes how to link these smaller files together to make up the clips and tracks in the project. The .aup file is in an XML format and can be opened in a text editor. The individual .au files are uncompressed, using 4 byte floating point numbers. The summary files have the same format as the audio files, but there are fewer of them.
The structure of the project files is designed to make editing audio faster in Audacity. By updating the .aup file, which is much smaller than the _data folder, Audacity can move audio around in the project without copying large quantities of data from one place to another.
